Flirting cheating?

So I have this friend who has a sister that used to work with my boyfriend. We were hanging out a couple days ago and she asks me "how do you feel about them talking?" and I said what? cause i had no idea, i know my boyfriend plays the facebook pool game with her but i thought it was just that as he plays with lots of people. Last night we got drunk and he asked me to send a message for him to a buddy and i saw her name pop up so ofcourse I looked at it, seemed like flirting which he started most of it.. she kinda flirted back.. lots of 😈 emojis.. a lot of it was over the game and some other convos not about the game. When my drunk ass saw it seemed like a lot. The next morning (today) I made sure he was asleep so ao could see his messages, when i look at them from a sober prospective it seems more innocencent but definitely flirty. I'm kind of hurt but am i over exaggerating? do you guys consider flirting cheating? am I being dumb?

edit we just moved in together so it's harder to walk away