What Causes Abdominal Pain And Short Menstrual Periods?

hi I am 23 yrs old female.I am suffering from intermittent abdominal pain in the left side anda also around the left pelvic region.I am 5'1 tall weigh around 49kgs. I have regular menstrual cycles. But from last 4 or 5 times my bleeding has reduced and I have bleeding for 2 days only. I have had an ultrasound which says no abnormalities.the doctor said I have Colitis but i am not suffering from diarehhae or bowel discomfort or constipation.please reply