I can't say for sure, but I love to speculate at times. It could just be that something bad happened that she doesn't want to talk about and that it has put a sour tone on things. It could just be hormonal changes. It could be that she has a problem and hasn't been able to bring herself to talk about it; being open and talking about issues can help (but don't try to force her - that would probably make things worse, and make her feel like you don't like her). She might be bored and want more romance. She might be angry at you for something you did, or something you did not do: finding out what is the trick, but if you're friendly enough, maybe she'll tell you (maybe not, but I'm convinced that being friendly/gentle in your encouragement to find out what's wrong will help in this situation). She might just be embittered toward men for some odd reason, and this may pass sufficiently for things to be semi-normal later on. She could be thinking that you won't give the kind of responses that she wants when she is responsive to you; just a thought - don't read into it too much. Be sure to watch her feelings and be respectful, and try to do things she wants you to do. Try to make her feel good.
How long have things been this way, just out of curiosity?
Anyway, that's my speculation and advice. Do well, and . . . yeah I don't have a lot of experience here, so, ask advice from others, too.
