Period is probably the most likely candidate. Either that or she's just trying to give herself an "out" if she decides she doesn't like you/isn't DTF. With the same action, she also gives herself an out if shedoesdecide to fuck you, because now she can rationalize it in her mind that she isn't at fault for slutty behavior (I don't necessarily think it's slutty, but women get that shit pounded into their heads that it is). "Well, I didn't really feel up to going in the first place, but he insisted!".