Girlfriend (f23) of a year cried and suddenly asked if I (m25) thought she cheated on me. Is her reaction normal?

About month ago when my girlfriend was staying over my house we were going through flights on her phone for me to meet her in North Carolina. We had talked about this trip months prior but she claimed she had spontaneously planned the flight without me because she figured I didn't actually want to go despite agreeing to go with her, resulting in me needing to get separate tickets. She does have ADHD and constantly forgets things but that was the first time I felt like she didn't really view the relationship as serious as I did.

While we were going through her phone looking at flights I wanted to take a closer look at the prices and asked to see her phone. She pulled it back and kept looking herself in silence. Maybe 10 seconds passed and she turned and asked me what was wrong. I claimed nothing was wrong I was just looking along with her and questioned why she asked. She said I got quiet and that she felt like something was off. I laughed it off and insisted both of us were quiet and that we don't have to constantly be talking.

It was fine until she suddenly started tearing up and asked if I thought she was cheating on me, saying I could go through her phone if I wanted. I insisted that I just had nothing to say and it never crossed my mind that she would do that. I refused to go through her phone and she went on to say that she would never do such a thing and went in for a kiss and then tried to have sex with me. We fooled around a bit but I tapped out halfway through because I was so taken back by the whole situation. We do have access to each others phone passcodes and she has admitted to going through my phone messages before, and she says I can ask her to see anything if I feel the need to.

Later that night I explained that I didn't think she was cheating on me but the way she reacted made me feel like something was up. She said she just assumed I thought the worst when she wouldn't give me her phone and that she would never hurt anybody like that because she's been cheated on twice in the past which we had talked about early in our relationship. There were never accusations from my end about her cheating but she had frequently asked if I was cheating on her for months until that point, and still does say she worries about me cheating on her.

We've met each others families and the relationship has been great otherwise, and I don't want to ruin a good relationship because I'm overthinking. I'm wondering if her reaction is a normal reaction for someone who has dealt with cheating and toxic exes, or if the guilt of doing something wrong overwhelmed her.

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tl;dr Girlfriend burst into tears and tried having sex with me after asking if I thought she was cheating on me. I feel like she just has had bad experience in past relationships and worries about cheating way too often, but I wonder if she's hiding something.