Hello. Sometimes even in normal individuals due to action of anticoagulants in the body during heavy menstrual bleeding clots can be seen.apart from this clots can be seen in certain uterine abnormality like fibroids in uterus,adenomyosis, endometriosis or hormonal imbalance like thyroid hormone imbalance or oestrogen-progestrone imbalance might also be a cause.this things need investigations like transvaginal ultrasound to ruleout the uterine abnormality. Hormone profile to ruleout any imbalance and most of the times the condition is treatable. So kindly consult a gynecologist for further management to ruleout this things.