What Causes Missing Periods, Stomach Pain And Cramping?

I'm 5'7 about 160 lb. Have had pcs asthma acid reflex nd cancer and diabetes runs in my family. I do smoke and occationally drink. I'm 19. And am having a problem; I have missed period since february no spotting.feeling nauseas mostly in the daytime. Stomach pains and cramping sometimes . I do not think I'm pregnant but please let me know what you think.