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You are having anemia and because of that weakness can be felt. You should get further investigated with peripheral smear examination, reticulocyte count, serum ferritin estimation, serum TIBC (total iron binding capacity) investigation, serum transferrin saturation to do further workup for anemia. You can be given an iron injection or PCV (packed cell volume) transfusion to raise hemoglobin level. Take iron-rich foods for increasing hemoglobin level like green leafy vegetables, jaggery, red meat, beans, etc. Take care.
For more information consult a hematologist online --> https://www.icliniq.com/ask-a-doctor-online/hematologist