What Causes Shortness Of Breath After Exercising?

Hi, my name is Meca, I have started this exercise movement at work, where you have to walk up and down 5 flights of stairs once for the first week, twice for the second week and so on. Yesterday was my third time and I am still overly short of breath, I had asthma as a child, I have had arrhytmias with ablation done in 2013, what could be wrong, oh and I have not exercised in 1 1/2 years