What Causes Chest Pain In A Child?

My 9 year old daughter has experienced chest pains for three years. We lived at the ER for the first year, and they diagnosed her with a lesion on her left lung. Spent 8 days in Millers Childrens Hospital, with 7 doctors, xrays and CT Scan. The lesion was gone. Was she mis-diagnosed or did it miracously disappear? So life goes on. The following year, the symptoms started over again. Whether it be physical activity or just sitting at her desk, the chest pains would come, and it was hard to breather. When you touched her chest, she told us it hurt, very tender. Back to the ER, but this time, they did EKG and Echo gram? Not sure if I m using the correct medical codes. Results were negative , they found nothing wrong with her heart. We continued with asthma testing - no asthma. Over the last two years, the pain has progressed. She can run for maybe 30 seconds and the pain starts. Just last week, she underwent the physical stress test (riding a stationary bike) and had to stop the test after two minutes - chest pains and hard to breathe. I am a single mom, and very frustrated. My daughter feels very defeated and her chest pains are now impacting her self confidence. Please help with some next steps.