What Causes Missed Periods And Cramps?

Hi, I'm a 17 year old girl I'm 5 ft 2 and I'm at a regular weight, I haven't came on to my period for at least 5 months now but I still get cramps, there's noway I could be pregnant due to my sexuality. I'm just wondering what may be causing this? I'm embarrassed to talk to my mother or doctor about this.