What Cause The Heavy Periods And Clots In Discharge?

my period was not due until the end of the week/early next week, but yesterday i began bleeding, and normally they are lightish periods, however it has been really heavy and i am having to change the pad every hour or so. I'm also passing clots everytime i go to the toilet and have passed 2 large ones since yesterday. I'm really worried because this is the first time ive ever had a period like this. i know docs always say is there any chance you could be pregnant, and yes i suppose there was a tinest chance, but if so it was only a week and a half ago, so not sure if this has anything to do with the bleeding? i had cramps/period pains yesterday but now its more twingy and sharp