Why don't i feel like having sex anymore

Me and my boyfriend started having sex pretty early on in our relationship, mostly because of me and the fact that i'm very sexual. For the last month or so, I haven't wanted to have sex. There's no logical reason why, and it's a drastic change from wanting to have sex every moment to not at all. I'm still very horny and i masturbate frequently but when it comes to actually having sex, it's not what i want.. What's wrong with me :/