How come this guy stopped talking to me after sex?

he was my best friend before any of this happened. every thing changed when he told me he liked me, after that we ended up having sex. then he stopped talking to me for 2 months. and with in that month i found out he got a girlfriend 3 day after we had sex. when i went on vacation he started to talk to me again for a good 3 months. then he stopped talking to me again. one month passed and we started to talk again, we ended up doing it again. then he stopped talking to me for a month. and this whole thing repeated it self for a good 10 months. each time he apologized for not talking to me, so i forgave him. later on people found out about us cause he told some of his friend but then he denied it ever happening. was he just using me? (ps. we was still with his gf when all of this happened). i mean he tell me alot when we do talk but is he just using me for sex? or does he like me? or what going through his head.