Like alijangra says, the best thing to do it's asking her if she has any issues in the moment. Sometimes a problem in your work, or a crisis in life, anything can put your libido down... even more being a woman with all these hormonal ups and downs we have!
Attention to the pill also, if you think she's diffrent since she started on it, maybe she's not noticing but it can change you, really. In my case it did, and I had to ask for another kind of pills. There are many options, so, don't worry about that. Just talk to her first and see what will be her reaction, the truth is always the best thing in any relationship.
Good luck and let us know of any progress.