It could be the BC.
On the other hand, I think it's pretty common that women get comfortable and their sex drives drop a bit. I've got a good 14 years on you and I've found that almost universally, once you get really, really comfy with someone, that edge isn't as strong.
Frankly, as long as the idea of sex isn't off-putting (ie., you dread it) you're fine. If you're really concerned talk to your Dr about it. She can change your BC, take you off of hormone BC (read: switch to condoms). [bTW, I went off the pill and switched to condoms and nearly killed FI with my renewed interest.]
If in point of fact you're turned off by the idea, there may be some imbalance that can easily be corrected with a change in BC or other hormone replacement, but your Dr can tell you for sure.