I've had periods when I feel out of breath from doing very little. So for me it is not PEM related unless I'm in a severe PEM state and have all the other symptoms to go with it. Over the years breathlessness was caused by several factors, below normal ferritin (5), low magnesium levels that when corrected with mg/taurine injections went away, and viral/immune issues. The only time a medical person asked me if I felt out of breath a lot was a biochemist who did my consultation for a panel of tests. My OAT in particular showed below normal markers. He did mention that the possible cause could be viral.