I am having very heavy bleedin twice amonth sometimes not even two weeks apart what could it be I have very large clots a lot of pain

Depends upon age. In my population (peds-young adults) the most common cause of menorrhagia, but not exclusive cause, would be dysfunctional uterine bleeding. In a older woman, other factors would need to be considered. This sounds like it's a significant problem, and you should see your doctor to have it accurately evaluated.
The . The most common cause for irregular periods are hormonal issues. First however you must check for anatomic issues such as uterine fibroids and polyps. These can be seen pretty well on ultrasound and i think i would start there. Your doctor also may want to do a biopsy of the uterine lining (endometrium) to rule out cancer- i usually do one on my patients over 35 who have irregular menses. If all these things are normal, then you can usually regulate cycles with something as simple as birth control pills. The Mirena (levonorgestrel) iud is also now approved to make periods lighter and there is a really nice office procedure called novasure that works well. If i had to guess, not knowing your age/medical conditions/race/medications, i would say fibroids. Let your doctor check it out with ultrasound and get that biopsy done.