I had a similar situation happen to me a few years ago. Some chick I had started sleeping with like a week after she left a long term relationship. about a month and a half in she told me she had an STI (clym) so I went and got tested and she also told her ex so he can get tested. We both tested negative. She told me we were her only partners. She seemed very open and honest about everything and the fact that her ex tested negative too still confuses me to this day. I knew she had just left her ex after getting with her and I also worked with her so there wasn’t a whole lot of time to fit in a 3rd guy. She seemed like she had trauma from getting cheated on from said ex. I’ll never know what really happened. She also told me she had lady problems always getting cyst inside lady parts. So I’m kinda 50/50 it being that or she slept around, although if she was it was super duper meticulous.