Loss of interest in sex

I've been with my boyfriend for 4 years and I've had the implant since we've been together. We moved in with eachother for university & ive been on my period for about two weeks and when I haven't been on my period I have 0 interest in sex. I'm still attracted to him but the thought of sex just doesn't interest me that much any more. What can I do about this girls and has anyone else experienced this?