Werid problem I have with sex?

Being sexually active with most of my relationships is very natural as it is; I have fallen in love with my current girlfriend - been together for over a year - but every time we have sex I feel as if I'm being used or I'm not even sure how to describe it. It feels as if sex is the only thing to are relationship but there is so much more and we both know it, but I still can't help from feeling weird? I'm not frigid, I like sex as much as the next person and I didn't have this problem with my last girlfriends I was a sex freak back then but i've changed. I know for sure I'm not gay. but what is actually wrong with me? :/