Why won't he have sex with me?

My boyfriend and I have been in a committed relationship for nearly a year and a half now. In the beginning, we could barely keep our hands off each other and the sex was beyond amazing. It seemed like we would have sex nearly every night, even while I was ok my period a few times. But for the last 3 or 4 months now, it seems like he just doesn't want to have sex with me anymore. I'm lucky if we have sex once a week. I know for a few weeks we were out of condoms and we won't have sex unless we have protection since I'm in college about to go to graduate school and he's planning on going to graduate school when I'm finished and we aren't wanting children quite yet. I just don't know what going on. I've even walked around the house in sexy underwear/bra hoping that would make him want to and nothing.. I'm not usually an insecure person, but I am beginning to wonder if there is something wrong with me that makes him not want to have sex with me. Has anyone has a similar experience or some advice?