As someone who works in information security and encryption
I hope this was not your work phone. While I'm sure you're smart enough not to send root passwords via SMS yourself, not everyone uses best practices, so you might occasionally receive text messages which contain sensitive information.
Customers will sometimes make you sign some sort of NDAs and some people liberally interpret "confidential proprietary information" as "we gonna sue you if anything we say including comments on the weather gets out". So, if your girlfriend perused through your work phone, any NDAs you signed with your blood become a liability. Not only to you, but to her also. I don't know the exact law term, but since she looked at the PIN over your shoulder, she was not authorized (good for you, you don't get sued except for negligence).
So, if this was your work phone, your colleagues really don't need to know your girlfriend just had a bit of "unauthorized access to confidential proprietary information"...
Now, I'll assume it was your personal phone with photos of puppies and the like. Let's review the facts.
You caught her twice going through your phone. You don't know how many times she did it unnoticed. Maybe she does it every day, maybe never, who knows.
Now, she made the choice to look into your phone, so what kind of tradeoff does that decision imply?
She gains: potentially information (but there isn't any), soothing her anxiety (can be important), and also she tests your response to her invasion of your privacy.
She loses: your trust, also you get annoyed, and this screws up the relationship.
She knew you would be hurt when you found out (because you told her) yet she did it anyway. This means she assigns more value to whatever benefits she gets by looking into your phone than she assigns to your feelings, or the status of the relationship. If this was not true, she would simply have thought "If I look through his phone, he's gonna feel hurt, and I want him to be happy, so I'm not gonna do that."
You, on the other hand, assign more value to your privacy than to her being reassured by not finding anything in your phone.
So, I proposed two hypotheses, the first in which she has no actual jealousy and is just acting like a jerk, and the second hypothesis in which she sincerely worries. You can invent many others, until you find the right one...
But she did not say she was sorry for looking into your phone, instead she seems to behave like she thinks she has the right to do it and disregard your feelings, which is a major tell.
In any case, I believed I was in the right condemning this behavior until recently, when a friend of mine told me his girlfriend did this all the time and he did not make much of it.