What Causes Severe Lower Abdominal Pain After Having Prolonged Period?

Hi . I have been having lower stomach pain and pelvic pain that hurts really bad it makes both of my thighs hurt and my buttock. I didn t have a mensual cycle last month in December and this month it came on earlier then usual but it was heavy for two days and then began to get back regular it stayed on for three weeks and just went off yesterday now im just having stomach, pelvic, buttocks and thighs pain really bad. Please help me as soon as possible I went to a doctor but they haven t let me know whats wrong yet and they want give me any kind of pills. I took a pregnancy test three weeks ago that came out posive and took another one three days ago that came back negative so now im very confused.