17 year old difficulty breathing

My 17 year old daughter had a slight case of pneumonia in the fall. Since then she has had tightness in the chest and difficulty breathing. This seems to happen mostly on rainy humid days, we live in the south. After a steroid pack, she has had a pulmonary function test which was fine, she also had 2 x-rays that were clear. Her doctor says it's heartburn or anxiety, but prilosec doesn't help and it happens out of the blue, when she's not anxious. I don't want to miss anything serious, could she have a heart problem? The steroid pack helped but the breathing treatment doesn't.