Girlfriend (24F) says that I (23M) gave her an STI, but I was a virgin before I met her

Hello all,
I am looking for some advice. Yesterday, my girlfriend mentioned that she had her routine health checkup, and she tested positive for a common STI. She said that she didn't test positive the last time she got tested, which was before we got together. She thinks that she contracted it from me.
The thing is, she is the first and only girl I've ever slept with, but she doesn't know that. I pretended to have had sex before so that I don't look like a loser virgin (that's how I saw myself).
I asked her if she had sex with other guys, after she got tested and before we got together, and she said no. I am inclined to believe her, but I still have my doubts. I don't think she is the cheating type, but I don't really know.
What should I do? Should I just forget about it?
TL;DR; : . GF thinks I gave her STI but I know I didn't because I was a virgin before I met her and I haven't cheated on her.


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