Hallow Dear, Negative pregnancy and starting of the periods have practically ruled out the pregnancy. If your menses were on scheduled time, the reason for this heavy bleeding seems to be either infection in the pelvis or presence of uterine lining (endometrium) somewhere else in the pelvis cavity known as endometriosis. I would not put hormonal disturbances in the top of the list of differential diagnosis because of the pain in abdomen, particularly during intercourse. However, the pain is more on one side, hence infection will be on the second place and I will consider endometriosis as the first possibility. Please report to your Gynaecologist. You may have to undergo certain investigations like ultrasonography, which will give some visual direct clue, hormonal profile, etc. May be you will have to undergo laparoscopy for confirming any pathology in the pelvis, particularly the endometriosis. Treatment of endometriosis depends upon its site, extent, age, whether pregnancy is desired and many other factors. Usually if the pregnancy is desired, the initial treatment is Oestrogen and Progesterone while if pregnancy is not desired, medicine called as Danazole may be used. Depending upon the extent and site, surgical removal or fulguration also may be attempted. In case of pelvic infection, antibiotics with anti inflammatory medicines will have to be started. However, you had pain only on one side, so if at all it is infection, it is more likely to be tuberculosis for which separate work up and treatment come in to the picture. The decision can be taken only after confirming the diagnosis, hence report to your Gynaecologist ASAP.