Sometimes things just don't seem to flow. I know that on a rare occasion my GF and I have awkward romps even though we've been committed and sexually active for 2 years. It passes, and, unless there's something going on in your relationship that makes you nervous, its nothing to worry about. That said, sometimes the unconscious picks up on things before you realize then intellectually. Consider your dynamic and take steps to change if you recognize a problem. Most likely, though, its NBD. Cheers, PS portishead rocks
I felt the same way with one of my old boy friends. I was concerned that he wanted move on the relationship and I wasn't ready to make a larger commitment, it turned things awkward. But then we talked and both realized that we were fine just being gf/bf and sex went back to normal (and sometimes even better than usual!).