Fatigue & angioplasty

My mother is 83 and was in very good health until last year.  She suffered some atrial fibrilation in August and was monitored in hospital but nothing was done.  She is on medication.  From then til December she did not improve in her ability to do activities.
This January she had a heart attack and was given an angioplasty for a 99% blockage in one artery.
The cardiologist says she is fine and all the tests show the heart is working fine.
However, she is exhausted all the time.  Sometimes even when she just gets up in the morning.
Can anyone tell me what could be wrong???????????

[Cleveland Clinic]

There are many other causes of fatigue, which are not heart related. But she could be fatigued secondary to the atrial fibrillation, or secondary to medications which are being used to treat the afib or even her coronary artery disease. It also could be related to renal dysfunction, anemia, diabetes, poor glycemic control, malnutrition or deconditioning. The differential is too broad without knowing her medical history, and results of lab tests and imaging studies.