Cheater/Cheatee, I'm The Cheatee

I had sex yesterday with the guy who took my virginity over a year ago. Today I was going through some things and found he has blocked me on a majority of things so I sent the ultimate search team to look into it for me, my friends!! The news I received was very upsetting!! He has a girlfriend, I was appalled by this news! We've been sending things for awhile now as well and I had no idea that she was even in the picture. I sent him a message asking if there is something he'd like to tell me and he didn't respond. I'm torn because I would have never had sex with him had I known. What should my next step be? See if he answers, keep it to myself, or go to her? I hold no interest in him anymore at all, it is his girfriend's wellbeing I'm thinking of. Please, please put in you input, thanks!!