Hi Syl-
Have you had any bloodwork for anemia? Joe was terribly anemic. At one point he even had to have a transfusion. This was prior to his last surgery, but the anemia continued for quite some time after his surgery. The valve repair took care of the major part of it, but he still sees a hematologist every 6 months, and will be on folic acid for the rest of his life. He was also on 3 iron pills a day. He's off those now. They didn't do much for his digestive tract, but they did help the blood get to a normal level.
Anemia can sap your strength and make you exhausted, plus it can make you feel out of it because there isn't enough oxygen in the brain. You wouldn't feel much like exercising if you were anemic.
Good luck, I hope you find the answer to the problem. It's terrible when you don't have any energy.