I would agree with dododo on heart damage causing weak heart. Weak heart or damaged heart pumps below capacity.Did the doctor tell you what the ejection fraction was ? Another definition of weak heart could be that the patient is suffering from heart failure. I have a weak heart but do work full time but on the other heart I don't have shortness of breath , a classical symptom of heart failure
It would seem that your doctor dien't give you very much info. Have you had heart damage? Do you have artery blockage? Do you have any other disease that would contribute to the weak heart?
Your doctor should sit you down and tell you the reason for the weak heart.
Tthere are many things that could contribute to breathing problems and fatigue.