Is it cheating?

Okay so me and my no bf were not together but we were talking and flirting and I had sex with a guy and I didn’t think I needed to tell him because we weren’t dating and he was still with his now ex at the time but this was like 8-9 months ago but just recently it popped into my head and it had been bothering me. I don’t believe I did anything wrong but why is it bothering me? Girls and I am bad person did I do something wrong why is it bothering me now please help