It seems that the symptoms you described such as an excessive and prolonged uteral bleeding with dark blood clots and lower abdominal cramps might indicate an abnormal condition called menorrhagia. Multiple organic pathologies can present themselves as abnormal vaginal bleeding, including lower level of platelets, lowered/increased function of thyroid gland, liver dysfunctions, hypertension, diabetes, and adrenal disorders. Pregnancy may be associated with vaginal bleeding that might be reported as abnormal for the woman in terms of timing, amount, or duration. Moreover, trauma to the cervix, vulva, or vagina could also cause abnormal bleeding. Structural disorders, such as functional ovarian cysts, cervicitis, endometritis, salpingitis, and myomas could be taken under consideration. Disbalance in sexual hormones, polycystic ovarian syndrome, vaginal infection, polyps, ectopic pregnancy, increased exercise performance or, simply, stress are recommended to be ruled out by a medical professional. It may be important to add that so called breakthrough bleeding might occur in patients taking oral contraceptives that have inadequate doses of estrogen and progestin for that patient.

However, It is advisable for you to visit your gynecologist, who might want to perform a bimanual pelvic examination, laboratory tests (hormonal and other), a transvaginal ultrasound and other interventions (only if needed) to rule out the exact cause of this abnormal uteral bleeding and prescribe an adequate treatment to have it stopped.