AITA for thinking my boyfriend might be cr*zy

my boyfriend thinks i did something in November 2025, with a previous partner.
i have known my boyfriend for over a year. We met a year ago at a bar. (we had an on and off relationship) He literally had an entire girlfriend that i didn’t know about until they broke up.
After they broke up, we started talking again. (even though he didn’t tell me he had a whole gf) ANYWAYS.
We started talking serious again in September 2025. (he was still doing weird shit and sending me pics with other girls)
In Nov of 2025 I was at the bar, and I ended up talking to one of my ex’s and he saw it, and accused me of cheating. (accusing me of more than just talking to him)
And while that sounds like a valid response… it definitely wasn’t.
He ended up giving me an STI (ch**m*dia) in May 2026. He says it was months before, and that it was “before we were actually dating” that he CHEATED on me!
but let me know! Am i the asshole for letting my boyfriend continue to shame me for “talking to an old partner and/or doing something with him” november 2025, when he gave me an STI in May 2026? because we weren’t “actually together”.