Blame the depo shot.
Depo will KILL her sex drive and it can take up to a year or more to get out of her system depending on time used, body weight, amount of body fat (it holds it longer). Remember, birth control tricks her body into thinking she's pregnant and totally screws with her hormones. If she never wants kids, she should get her tubes tied... it can be reversed if she changes her mind later in life and is just as effective as "normal" birth control and has none of the side effects.
Went through this with my wife... same with a buddy of mine and his girl... took 1.5 years for it to finally clear her system. (There were other signs, not just a lack of sex). Thank God she was aware of it and "tolerated" my hounding.
