He didnt tell me he had a girlfriend?!??!!!?

so i was at this rlly fun party last night, and this guy at my school named jd who i dont tlalk to much at school was tehre. we started talking a lot and flriting and i realized he was really cool. we were all outside and he decided to give me a piggy back ride, and i laughed an accepted his little 'gesture', and he was carrying me around and i was all "ha your pretty fun:)" and he said "you are too. what do u say we get out of here?" and i was like, (being the flirt i am) let him pull me into a room and he started totally kissing me, and i didnt really mind at all. then after that, i hear from my friend liliana that hes dating one of my friends! not a close friend, but still friends. i feel bad, but then again i didnt know. is it his fault?? or is it mine for being a ho!? help D: