I read carefully your query and understand your concern.
If I were your treating doctor I would come out with some possibilities and would order some tests to exclude most common causes including :
-Anemia.You should run a complete blood count
to evaluate red blood cells and hemoglobin
.A hemoglobin under 12 mg/dl means you are anemic.
-Diabetes . You should run a sugar blood test
. A fasting glucose over 126 mg/dl indicates diabetes.
-Thyroid conditions -Hypothyroidism
(underactive thyroid gland).You should test your thyroid function :FT3,FT4,TSH.
Low FT3,FT4 and high TSH indicates hypothyroidism.
-Cardiac disease.You need a complete cardiac work up including physical examination ,ECG,Stress test
I suggest to consult a physician and get examined.
Hope my answer has helped.