Ex And sex ⁉️

Diana
I need advice or opinions. So I was dating my boyfriend for 4years within these past 2 months we were on and off. 
He spoke to new girls I spoke to new guys. 
Recently we decided to work on things again and I asked him DID YOU SLEEP WITH ANYONE he said no. & of course I beilved him. I found out he did sleep with this girl and now she's out of his life and he's still trying to fix it with me but idk if this is a good idea now because he LIED about sleeping with this girl and then slept with me !!  Am I overreacting ??. He said he lied about it because he just wanted to forget about her since she was a "rebound". 
😪😪😪😪😪