I don't think it's a matter of simoly being horny one moment and then not because of tiredness or whatnot. It's like my whole attitude towards sex does a complete 180.

One moment I'll be horny out of my mind, sexting my boyfriend if we're not together or having lots of sex if we are. I'd get all touchy touchy with him and purposely act sexy in front of him. 

Another moment I'll be completely repulsed by sex, looking for excuses to avoid it or doing it just for the sake of it and telling myself "it will be over soon". I'm repulsed even by my boyfriend touching me or making sexual jokes with me, things that I don't mind otherwise.

I'm wondering why this might happen? I haven't identified a trigger yet, it seems to happen randomly.