This could be one of two very different things, and it's near-impossible for us strangers on the internet to tell which it is.
Scenario 1: the one-sided relationship
The first possibility is that she has a very different idea of what this is than you do. It sounds like you're developing some pretty strong feelings; it may be that she just thinks of you as a "friend with benefits" or something of that nature, and this is just a casual hookup to her. She comes over, she gets to chat about her day, vent a little, chill out with a movie, and scratch an itch or two. Enjoyable, but nothing serious.
If this is the case, your best bet is to politely but firmly back out of the relationship. There's no need to cut all ties or stop seeing her, but you need to set a few boundaries. First, you need to set a boundary in your own mind: she is not your SO, and this is not the sort of relationship you want. Allowing yourself to become more attached will only make it hurt more when she explains that the sex is going to have to stop because she's seeing someone and it's going really well.
The second boundary to set is with her: let her know that you don't think things can continue the way they've been going. If you must explain why, just tell her that you don't think it's a good idea for your emotional state, you're not doing very well with the whole "casual" thing, and it's probably better to just call it a day before someone gets hurt.
She may know that it's one-sided, or she might be under the impression that you see the relationship the same way she does. Even if she does know how you feel, she might not be deliberately taking you for a ride; she might just not know how to address it, or be hoping that you'll realise and get past your feelings. Don't assume malice where misunderstanding or awkwardness-aversion are just as plausible.
Scenario 2: the awkward beginnings
The second possibility is somewhat happier: she may be in a similar position to you, just a little behind. It's possible that she has feelings for you, but she's not sure how much, or exactly what they mean, or what she wants to do about them. Maybe she wasn't expecting these feelings to develop, maybe she's conflicted because she saw you as a friend and now this is happening, maybe she used to see this as a FWB thing but now it's something else. If this is the case, then her asking you not to tell anybody is simply a precaution and a defence: she knows that as soon as you say "we're a couple", people will get excited - which she may find a little overwhelming - and start asking questions - which she may not be sure of the answers to.
In this scenario, you'll want to have a conversation about maybe doing some other things besides your usual Netflix-and-chill routine, and evolving gradually into whatever style of dating you two want to engage in. Take it slow, and mostly keep doing whatever you've been doing because it seems to have worked so far.
How to tell the difference
This is generally viewed as the hard part. The solution is actually very simple, but in relationships, "simple" is very different to "easy", and it's often the simplest parts that are hardest. This may be one of those times, because the simplest way to find out what's going on is to talk to her about it.
You've said you're not great at talking about your feelings, and you suspect that she might not be either. I won't lie to you - that's not going to work in your favour. On the other hand, it's absolutely a thing you'll have to figure out if you want a lasting relationship, so you may as well bite the bullet and try it now.
You said that she asked you to "keep our thing secret for now". That's a good start - it gives you an opening. Just ask her - casually, and gently - "hey, I was just wondering about what you said the other day - I don't mind keeping it a secret for now, but what sort of things might make you more comfortable with people knowing?"
You're not asking for a timeline here - do not mention dates or timeframes. What you want is more like a roadmap; stages along the journey from "friends" to "in a relationship". If she says she doesn't want to make it official until you've been on a few proper dates, or something of that kind, then that's something you can work towards. If she says something more vague, like she wants some time to think, or she's not sure what she's feeling, then maybe you could suggest - again, gently - that you go on a few dates, just to see how it goes, and maybe that'll help her work it out.
It might be that the conversation doesn't really bear much fruit. That's fine; if she's really reluctant to talk about it, just let it be and move on to safer conversations, and circle back to it a week or two later. You may have to do this a few times, but that's alright as long as you make a little progress each time the conversation comes up. By "progress", I mean... well actually it's hard to define clearly what "progress" means, but in general if you wait til she's gone and you're alone, and you ask yourself "is anything different to the last time we had that talk?", chances are you'll know the answer, even if you're not willing to admit it. It might be that she's opening up more about her feelings; it might be that she's admitted she feels something for you; maybe she's started taking the initiative and suggesting date-like activities you could do together.
On the other hand, if these conversations start to get repetitive, or if she keeps shutting you down whenever you try to ask, then it starts to become more likely that you're actually in Scenario 1 and she's trying to keep you from figuring out so she can take advantage of it for a little longer. It might also be that she's just really unsure and averse to talking about it, but to be frank, I wouldn't advise starting a relationship with someone who's that unsure whether they want to be with you, and unwillingness to discuss her feelings with you is rarely a good sign, so either way your course of action is the same: revert to Scenario 1, set boundaries, and get out.
Some small observations
Two things I want to pick up on.
First, when you went out with some friends, she asked you not to mention it to anyone. That indicates that she's aware that you feel there might be something to announce, which suggests that she is at least partially aware of your feelings. I'd advise against reading into that, but it's worth bearing in mind.
Second, you seem to be heading into classic new-relationship-overthinking territory. You're trying to figure out how she's feeling, you're coming up with scenarios ("she might have had feelings at the beginning, but due to the fact that I haven't told her that I love her, her feelings have vanished"), and generally analysing the whole thing in great detail. You've even come to ask the internet for advice and assistance in how to proceed.
I'm not criticising you; this is normal, and just shows that you feel pretty strongly about her. All I'm saying is that by doing this, you're complicating things, and it's making you miss the obvious solution: if you want to know how she feels, ask her. Be honest, be open, and be gentle. Tell her you've been thinking about your relationship, and you're not sure what it is or where it's going, so you'd like to get her take on things. She can answer that question way better than any of us strangers on the internet.
