Reasons for period cramps, but no period?

Well, I haven't had my period since January. I normally get it at the end of the month, so currently I'm ten days late. I've taken three pregnancy tests, all negative. But I get cramps almost every day. Like menstrual cramps, and I feel like I'm going to start, but never do. Should I be concerned? Could I still be pregnant? I'm kind of worried. This has never happened before, and my period is never late.