Its been 3mths now and I havent had a period!!!! What could be wrong?

ok i missed the whole mth of jan., feb., and know its going to be the whole mth of march! I have never had this happen to me, i have took 2 home pregnancy test both came out neg. and then i have already been to the doc. office and they took another and neg. also! so im a little bit confused here if i am not preg. what could it be i have been feeling really bloated lately, tired, and lower backaches that i cant even sit down sometimes cause it hurts! so can anybody tell me anything about this situation what could be wrong?