My 18 Year Old Daughter Has Been Sick Since Dec.

My 18 year old daughter has been sick since Dec. 17 she has lost 15 pounds and has had the worse nausea ever sometimes throwing up and a pain in the right upper abdomen area, headaches are bad as well, She has been give an endoscopy and it was clear, ultrasound clear they wanted to do an MRI but haven't and a CT if they don't find the cause. What can this be?