Green Spot right above during period?

Hey guys! So when I get my period about 3 1/2 inches under my belly on (above the "mound") there is a light green spot under the skin about the size of a quarter. So im guessing its where the bladder/uterus area is. Do any of you knwo what this is or has anyone ever experienced it? P.S. It goes away when my period is over. Sorry if this was a little too discriptive!