Is it normal that i had sex with my best friend and regret it?

Okay, so my boyfriend and I broke up recently, and I was really feeling down. At first he said we were on break and my best male friend Chris was having a party. I hadn't seen Chris in 3 years and I figured I could probably use a little pick-me-up. So my friend heather and I went to the party, and Chris and I kinda liked eachother for awhile. He went to his room and I decided to go in there just to catch up with him, and have an innocent conversation. I had no intention on having sex with him. Well as the night went on, he and I started flirting. Well he and I went back to his room, once again just to talk and he was laying down, I was sitting on his bed and he asked me why I was sitting up, apparently i was too high up (?) So i laid down, not thinking anything of it and before I knew it, we were having sex. Not gonna lie, it was really good. But I felt bad about it because my boyfriend and I weren't completely broke up and I'd already fucked someone else. Not only that, but Chris and I are best friends and I don't think it should've went that far. Everything's normal between he and I, we're still really good friends..but should I be considered a slut for doing that? Should I feel bad about having sex with my best friend? Should it make me kinda happy when I remember it though? I'm so confused!!