My period is extremley heavy, and big clots. Used 7 super tampons alreadytoday. What is causing this?

Need to see doc. When periods are that heavy it is important to see your doctor. This is not normal. Fibroids, polyps, bleeding disorders, endometrial cancer, cervical cancer, hormonal imbalances all should be investigated.
Thyroid disease. Also, you should get your thyroid checked with simple blood test.
Heavy periods. Normal menstrual flow lasts for about 7 days and needind less than 8 pads per day. A change in menstrual flow could be due to different conditions, hormonal(lack of ovulation), structural(uterine fibroids), to cancer of the uterus. Any of these conditions should be evaluated by your doctor.