daughter's chest symptoms

My daughter has been having symptoms of something that started about 2 years ago. She started coughing up mucus. At first I thought it was allergies but it has progressively been getting worse. She now coughs up mucus all day long, her chest hurts, its hard for her to breath, she is very fatigued and her knees and other joints hurt but mostly her knees. We have been to see our regular doctor, ENT, cystic fibrosis specialty clinic. My daughter is now 15. So far she has had 2 chest xrays which showed absolutely nothing. They have ruled out cystic fibrosis with 2 sweat tests, allergy tests came back completely negative(blood test), lymes test-negative, asmthma has been ruled out, vocal cord disfunction has been ruled out. She was extremely constipated. Because her knees have hurt so bad she walks with a strange gait. When she would go to the cystic fibrosis clinic she would do the breathing tests there before seeing the doctor. She did have a hard time with these tests. Many times she couldn't breath out the entire way without coughing. The doctors cant seem to figure out what is wrong with her. She's very frustrated because she feels that no one can help her. I wanted to know if you have any ideas for me what this could be. Her biggest complaint is the chest pain, not being able to breath well and the general not feeling good most of the time. Should I bring her to the Mayo Clinic? Do you have another avenue we could follow? I really need some help.