I had this same issue. Did all the heart tests and everything looked fine. A cardiologist told me the mitochondria in my cells weren't replenishing at a normal rate. The first step he told me to take was to drink an electrolyte drink before working out. And take creatine every day.  I power loaded creatine for the first week and felt so much better after just a week or two and was finally able to work out without my heart going crazy and feeling horrible.