Menstrual for far too long?

My 18 yr old has severe menstrual flow. First was for 16 days, then 21 days, then normal 8 days;now 40 days. The doctors (2 different ones) have done nothing. One said it could be from the tampons she is using! One did a paps but no results as yet. We are at a lose and are currently searching for a female doctor that can help. I am concerned as I had total hysterectomy for mutliple ovarian cysts at age 27.This is not normal and we ask if anyone out there can advise what we should do.As she is a University student I did not go to the appointments with her; but thinking that this might be a slight problem and I should attend the next visit. Answers would be greatly appreciated.