I have been having pain on my uterus and my lower stomach for about two months, it only calms down when i have my period, i've taken two pregnancy tests both negative, what could it be?

It is hard to say what that is but is may be intestinal pain due to constipation or endometriosis but endometriosis usually is also painful during your period. Endometriosis pain seems like regular period cramps, but they can happen any time of month. You may also have cramps and pain in your low back and stomach below your belly button. Best to see a doctor and be examined.