What Causes Difficulty Breathing While Having Controlled Asthma?

I have had chest x-rays, ultra sound of my heart anand carotid artery, heart catheritization, and stress tests and doctors have found nothing to cause my increasing breathing difficulties. I've had asthma since 1976, 60% lung capacity, take Advair 250/50 and Flonase.
My increased breathing began about a year ago. My lungs are clear and my asthma is under control. Is tbere anything other than heart or asthme that can cause heavy breathing from just walking to the bathroom 25 feet away?