large clots goes with a heavy period, per my doctor's advice. when i had this problem my doctor had to perform a d&c on me which is scraping the lining of your uterus. when it gets thick for some reason it causes you to bleed heavy.
after this procedure you can try to get on some form of birth control pill to regulate your period.
try taking iron pills so you dont have to worry about having a lot of blood lost.