Well, have you tried asking her? It would definitely be an awkward conversation, but maybe you're reading too much into it (e.g. she has a yeast infection or something and she's embarrassed).
But the big question that your post raised for me was this: why were you abstinent for 3 years? Was that a mutual decision or was it more from her end (the latter sounds more likely to me, given that you think you should be going at it like animals)? Maybe she feels like she made the wrong decision by becoming sexually active. If that's the case, the two of you should have a discussion about what your sex life will be until you get married (if that's your plan). Then you'll have to decide if you can live with whatever consensus the two of you reach, or if it's time for you to move on.
Finally, another possibility is that you're just bad at sex. No offense intended - most younger / less experienced guys are (if I'm making an inaccurate assumption about your experience prior to your three years of abstinence, my apologies). [Side note - before you men get all up in arms, I'm not trying to be sexist here. Younger girls are probably pretty lame at sex too, but I think it's easier to give a guy an at least minimally enjoyable sexual experience. Guys have a tougher job trying to please a woman.] Especially if your girlfriend was a virgin, she may have found that sex was just not as enjoyable as she had been expecting. If that's the case, then you're in luck - the best solution is more practice! Get her to tell you what feels good, but also do your homework - read some of those stupid sex tips in magazines, watch some porn, etc - a lot of these will be lame but you'll at least have an idea of where to start, what to try, and then she can tell you if she likes it.
Clearly, the theme here is that you need to talk to her. It sounds like you otherwise have a good solid relationship, so hopefully it works out for you. But don't shortchange yourself either - a relationship between people with vastly mismatched sex drives is bound for trouble, so there's a chance you might just have to recognize that and cut your losses.