What Causes Missing Periods, Cramps, Back Pain And Sore Breasts?

For the past three months I haven t had my period. I get the cramps, PMS, back pain and sore breasts when it s time for my period but then it s a day or two of brown discharge and then it s over. I have never been sexually active so I m not worried that I m pregnant I just want to make sure there is nothing wrong with me.