What Causes Irregularities In Menstruation?

My cycle has been on for 20 days today. My cycle usually lasts for about 4-6 days. Im use to having irregular periods but not like this. I havent had a cycle since Feb, didn't have one March and April, neither have I had unprotected sex. Ive been pregnant before and I know the symptoms. I have several tests and Ive been to the doctor and was negative also. Ive been having abdomen and pelvic pain. I use the bathroom alot as far as to urinate or have a bowel movement, but the bowel movement Im use to because I'm lactose and always had problems with digestion. I get tired and dizzy. Frequent back pain every once in a while. I get fevers once in a while and headaches.