What Causes Chronic Fatigue And Discomfort Post Long Walk?

I am taking 25 mg metopolol succer once daily, post double bypass 9-11-2012. I was doing fine with cardiac pt and then I got real tired and sob when walking. I have those symptoms still. I recently lost my daughter (47) years old). I had a Heart cath in March 2014 which showed the bypass arteries in good health. Sometimes I just feel there is something else. Originally the ekg, ultrasound and the chemical stress was all normal; dr sent me for heart cath which showed the blocked ateries. I still feel tired and have trouble walking a long distance.