What Causes Pain In The Lower Right Abdomen?

I have pain in my lower right abdomen, like where my ovaries are, and it goes into the right side of my lower back. when i am sitting up straight or leaning foward that is when I feel it. When I lay down or lean back it stops. I also have not had my period in 4 months, but have taken numerous pregnancy tests and all negative; besides that there is no way I could be pregnant I have not had sex within those four months..any ideas??