Would you believe him?

This guy I used to date told me he thinks my best friend is envious of me and she isn’t really a good friend. She spoke about me bad to him when him and I were not really on good terms. He told me something that I told her before but it’s not that personal abt me but it is a fact. He said that they did something sexual. It really makes me question many other events idk what to think. It didn’t happen while me and him were still seeing each other but literally shortly after that, gotta be within the same month or the next month so it’s like she’s been plotting?