What Causes Shortness Of Breath During A Physical Activity?

I have had shortness of breath for about a year. I was a bike rider and it wasn t unusual for my brother and I to ride on the bike trail 35 miles a day three times a week. I have always been somewhat overweight, but was always very active, and all of a sudden I couldn t walk the 50 yds to my mail box and back without being very short of breath. I have been to heart Drs. and they said I needed a cath,then stent, then pace maker. Went to a lung Dr. and he said my shortness of breath is imaginary, then he sold me a $47 a month puffer. I have been to Cleveland Clinic as my stomach Dr. said I had something wrong with my esophogas. Cleveland Clinic changed my blood pressure medicine to one that would make the esophagas relax and it has done that but I still am short of breath. Have been sleeping in a recliner for a year, mostly. Any advice on what kind of Dr. I need to go to next.