Why am I so horny?!

I have been in a relationship with my boyfriend since I was 16, now I'm 21. I gave him my virginity and haven't been with anyone but him. Through all those years I never really felt attracted to other guys but since he joined the military 3 years ago I recently am finding other guys more and more attractive. I haven't done anything with anyone but I'm just curious as to why is this happeneing now? Is it because he's so far away?