scared..

I had sex with someone when me and my boyfriend broke up and last time this happened I told him and he never let me live it down. now I haven't told my boyfriend and the guy I fucked seems cranky and like he may snitch on me. ik I sound like a whore. I'm scared and idk if i should tell the truth or get on my boyfriends phone and block this other guy on everything..ohh tbh I feel bad..but my boyfriend he also told me lies about before we got back together.. i confessed it all to my mom and she said it isn't my boyfriends business what I did when I was single.. any advise?