Why do women just stop having s? Or is that just me?

I've been dating my boyfriend since I was 15. We didn't get ually involved until I was 17, and didn't have until I was 18. When we first started doing things I deffinately had s. After a while I didn't anymore. Since we started having almost a year ago, absolutely none. I started the pill then but I had this problem before anyway. There's just nothing! I get kind of.. twitchy? That's about it. I feel like my ability to just...broke!