Hi, Knowing your age would have helped. However the following are common causes at any age: 1. Mild hormonal imbalance; very common; difficult to detect, leads to pelvic pooling of blood, slight increase in size of uterus and bleeding with pain. 2. Pelvic infections; again common; usually associated with some discharge from vagina and may have fever. 3. Uterine fibroids or adenomyosis: non cancerous tumors that cause bleeding and pooling of blood in the pelvis leading to pain and heavy periods. 4. Endometriosis: some menstrual blood gets into the abdomen and starts bleeding there leading to pain and sometimes heavy bleeding. the pain in this case is usually severe. I suggest that you wait for one or two cycles if the symptoms are not very severe as hormonal balance tends to correct itself after regular exercise like walking twice a day and eating plenty of fruits like tangarines, and pomegranates or any other seasonal fruits that are not very sweet. Check to see if you have any additional symptoms from the above mentioned ones. A pelvic ultrasound helps to diagnose the conditions and in cases of infection a vaginal examination and cervical and vaginal swab test to detect infection must be done. Hope you find this useful. Thanks for using HCM. Have a good day!