Being friend with the "side chick"

I was with my ex for a little over 4 years, i broke up with him. A couple months after, I found out that he was cheating on me. I only found out because me and the girl became friends through my best friend. One night we were talking about out past relationships, and we found out that we were both with the same guy. We both thought we were the only girl in his life.😂 he wasnt really into social media, but now it makes perfect sense) me and the girl are still friends, is that weird?