What Causes Missed Periods, Bloating, Cramps And Pain In Lower Back?

Hi, i last had my period on the 12th of December 2010. it hasnt come as yet and its mid march already! i have been getting some menstrual cramps since january, but not as intense as my period. but it comes and goes. i get these headaches sometimes for no reason. i think i have gained a little weight on my tummy. it looks a little bloated than usual. i took a pregnancy test about a week ago, but it was negative. i have some backaches too, especially the lower back. i have the urge to urinate more than i usually do. yea, thats about it! NO food craving, NO tender breasts, NO sickness. but where is my period and why the cramps every now and then? i'm confused. please help. thank you