Sex is all I think about!

I'm constantly thinking about sex (when I'm not having it)... When I first got with my boyfriend we did it everyday sometimes 3 times in one day! We've been together a year and now we only do it every second day if I'm lucky... Is he not attracted to me anymore? I always try new things to spice it up but he sometimes isn't so keen, his on Zoloft would this play apart in his sex drive??!