10 year old breathing difficulty

My 10 year old has been diagnosed at a childrens hospital with asthma about 4 years ago As an infant I had been given an inhaler and mask for her because she was wheezing after throwing up her formula which she did often . Then she was fine for years until she started to have tonsilitis at age 3 her tonsils and adnoids were taken out She always has what we refer to as a cloggy throat Always clearing her throat filled with plegm and I mean every morning she wakes with this and had this prior to the tonsil surgery. She at age 6 starting have extreme breathing difficulty rarely ever with a cough or wheezing. She was tested for allergies and asthma twice She takes singulair albuterol inhaler and xopenex viles in her nebulizer according to the recommendations of the doctor . Each time she has a spell of not getting her breath she also experiences heart pounding episodes Also after strenuous exercise occasionally this heart pounding occurs. She has some test at cardiologist and then they were going to monitor her heart for 30 days at which time the heart pounding subsided for about 2 years She is now going through an aweful ordeal When the drs take her pulse ox it is at 98 or 100 percent and yet she feels as though she can't breath .She took prednisone for 5 days and is on her 7th day of cefzil for infection just because this is what the dr thought she needed She has never gotten much relief with the albuterol or the xopenex which makes me wonder if she indeed has asthma or could it be something else I would really like answers if anyone can help or give us guidance or advice on this situation. It is very scary for her. And we are working on easing her anxiety when she feels she can't breathe .