4 years ago I had a back operation and 2 months later I had a hard time breathing. Every time that I exercise or increase my heart beat for a couple of days I find it hard to breath. When I take my dog for a walk I have to keep my pace down to around 2.5 MPH or so and if I go to 3.5 MPH I could be down for a week. I also find myself yawning a lot. This has been going on for almost 4 years. My heart is OK and I pass the entire lung test and I seen all the head doctors so it is physical. Anyone ever have or heard of this?