What Causes Heavy Menstrual Bleeding For More Than A Week?

Question: I have a 15 yr old daughter who had her period nearly everyday for over a year. It was not spotting it was heavy bleeding and would not have any bleeding for maybe 4 days out of each month. Chronic cramping and migraines associated with this. Her dr. has just put her on BC pills they tried the strongest one on the market first and this shortened the bleeding days to maybe 2 weeks a month but her periods were extremely heavy and painful and full of clots. They switched her to another one with lesser hormones in it and she has one period a month however it lasts about 2 weeks heavy with blood clots and a lot of pain. Also if my daughter forgets to take her pill (doesn't happen often) which she takes before bedtime she will be bleeding by the time she wakes up.
My daughter also has joint pain everyday, all over her body, she has either bad constipation or severe diarrhea with bleeding, never in between. Negative for crohns/UC. She also gets bad migraines, which being on magnesium daily has somewhat helped. She is also going bald. Her TSH levels have been normal and her Iron is high, not low. Her vit B levels are okay, but she takes a B complex daily as well as hair and nail vitamin and a multivitamin.The other thing that concerns me is that she catches things very easily. It is as though she has no immune system. A cold turns into pneumonia, if she gets a stomach bug she ends up in the ER needed IV fluids because she will vomit so much in a 24hr period that she can't even keep water down. She gets tonsillitis and strep throat all the time. I am constantly bringing her to the Dr office or the ER and I really would like to find the root of the problem so she isn't always in pain or going to the Dr's office. She has severe depression and I would too if I was always in pain and bleeding. I do not know what I should have her PCP look into anymore. Auto immune disease? endocrine? Rheumatologic? Any suggestions would be helpful.
She has had uterine ultrasounds which most were normal, she does get ovarian cysts. She has not had an internal ultrasound as she is still a virgin and they will not do that on a virgin.
CBCs are grossly normal, iron was mildly high when last checked during one of her heavier periods. TSH normal which has been checked twice. She has also had appendicitis with an appendectomy on August 1, 2015

She gets sick with dairy products but is not lactose intolerant. She drinks plenty of water and eats a lot of fruits and vegetables and little meat.
Could she have endometriosis