I recommend acknowledging the situation, de-personalizing the issue, and deflecting with an observable effect which I present as trivial.
I have OCD, and am compelled to carry out some bizarre-seeming behaviors or else face rapidly increasing anxiety. If someone notices that I seem tense or anxious, and that's the reason, I often don't really want to get into a full description that it's because I can't engage in some arbitrary behavior. Instead, I go through some variety of the following:
Acknowledge that the other person has indeed noticed something real. They've perceived my anxiety, and telling them that they are mistaken changes the focus from my affect to the other person's perceptiveness while also making me seem furtive.
Make explicit that the anxiety primarily comes from and is about me, not them. If not for my OCD, these issues would not arise. Regardless of the specific context, the anxieties spring from my psychology and not (meaningfully) from my environment-- if someone does something that my OCD demands I respond to in some way, it's not that the other person has done something wrong. It's just that I have a non-rational compulsion to respond.
Especially with things that can seem like personal affronts (like making someone feel anxious), it's easy to feel blamed in some way by the situation. And since that blame is inappropriate in these situations, the soul of tact here is to relieve the other person of any sense of responsibility or blame. Even when other people can do things to ease your distress, and may be willing to do so, they aren't really the cause of anything, and so there's nothing they should feel bad about.
This was the case with your Aunt's presence: even if she makes you particularly anxious, the description in the OP didn't make it sound like the anxiety was because she was there so much as because an large enough, unfamiliar enough group was there. Perhaps saying "because you are here" to answer your Aunt's question wouldn't have been a lie, exactly, but it wouldn't have been all that accurate, either.
If the goal is to get the conversation to move on and stop the other person from devoting a lot of attention to something they can't influence much, I like to give them something readily understandable/observable and then clear permission to not worry about it. I believe that this helps to mark out a noticeable piece of my affect, links it in the other person's mind to the arbitrary issue coming from me and me alone, and then links that to an express indication that it's not a cause for concern.
- Optional Step: Thank the other person for their attention and
This can do a lot to defuse any lingering tension or bad feeling in the other person. It acknowledges their attentiveness to you (that's how they noticed something was off), validates their observation again, and shows that you appreciate that attention and that they care enough to want to understand why you're distressed and help you feel better.
Putting it all together, here is a sample exchange demonstrating how I might try to accomplish each of these in a situation like the one described in the OP:
Aunt: You seem tense. Is something wrong?
Me: Oh no, nothing is wrong. But you're right, I am a bit tense [acknowledgement]. I don't really know why, but sometimes being with a group of people in a social setting makes me kind of anxious [de-personalization]. It can be noticeable when my anxiety flares up like that (clearly, since you noticed!), but it's not a huge deal [highlight the anxiety, then minimize its importance to help deflect]. Thank you for noticing and asking after me, but it's not worth derailing our conversation or using up our limited time together [thank the other person].
And then, if you're able, redirecting the conversation to another topic can help transition into not focusing on your anxiety any more.