Also, just ask her in a few days what is bothering her. Often being emotionally fragile, and after sex they are in the most precarious of states...often one little off-the-cuff comment you make could upset her.
Often these paranoid and guilt driven ladies take comments out of context. Or perhaps they just do not understand your point-of-view. But regardless as Missy suggested, you could talk about it.
Girls love nothing more than mindless drivel about their "feelings."

One more thing, girls almost always want more than sex. You can have sex with the biggest slut in your geographic area and they can come out and say, "all I want is pure carnal sex." She would be lying to you. Almost always they want at least some emotional attachment and that comes in the form of a relationship. Now they may not even feel comfortable considering it a relationship, and it may not be their only relationship, but mark my words ---they want a relationship and you better get away from her if you do not.