happened 3 times to me
1) got a bit crazy on a super low carb diet (about 20-40g carbs a day, for over a year, which is quite extreme), which resulted in hypothalamic amenorrhoea. (Ie. low thyroid, resulting in no periods). And no girly hormones = no sex drive. However, this also happened to me when I was 18 and dropped a lot of weight - it doesn't just happen on low carb, but any time when your body feels it's getting enough food. Now I'm on low carb, high protein and high fat, which is great.
2) Just stopped fancying my BF, after 3.5 years together. Sad, but it happens. Although probably not the case with you if you have no desire for other guys
3) When I was on the pill. Urgh, never again!
However, the female sex drive is a curious beast. If sex feels like a chore, it's hard to get in the place, mentally, where you want to do it. Maybe ask your partner to drop the pressure for a few weeks, and then only have sex/do sexy stuff when *you* actually want to.