My Period Came Yesterday , It Is Very Heavy And

My period came yesterday , it is very heavy and i see alot of clots. Yesterday in the shower, i noticed it was a little slimy. Today i feel fluttering in my stomach on the lower left side. Ive also noticed i gained weight, only in my stomach and have begin to get stretch marks as well. I have gallstones if that helps, ive also took pregnancy tests, 2 - they were both negative. What could be going on ? The fluttering started about 30 mins ago and it wont stop. I've been urinating all day, I went about 7 times.