Period Question!!

my period used to be a good 5 days with a regular flow and some cramps here and there - this was the norm for me for about 7 years. But for the last three months my periods have been 10+ days, heavy flow, and excruitating cramps. I just want to know why, my doctor has no explanation and says that it has to do with me weight (I weigh 180 and a person my age (18) is supposed to weight 145 but I really don't see how it's my weight if I've weighed 180 for the past 2 years soooo....) I'm desperate for some answers. I want to start birth control but all the side effects scare the hell out of me..