I didn't cheat

I broke up with my boyfriend about 4 months ago. We've been good about the whole "friends" thing. Recently we have been talking a lil more. Friday we met up for lunch and he and I talked for hours and hours and it was wonderful. He kissed me and said he wanted to get back together... and today he came over and things got interesting... we ended up having sex. During the time in which we were broken up I had sex with another guy. My ex asked if I had slept with anyone else, I said no... I had a dream last night that he said "I'm so glad you didn't adventure w/ any other guys." But I did and I feel guilty as hell... do I tell him?