@Mammmood - I’ve heard that people who have gone through chemotherapy can suffer from a low red blood count. This is because the chemotherapy can affect your bone marrow, and your bone marrow is responsible for creating red blood cells.
Your doctor will probably draw blood a lot during the cancer treatment to make sure the red blood cells are not getting too low. If they are he may prescribe additional medication to boost the red blood cell count or at least modify the chemotherapy treatment somewhat. He may even stop it for awhile to let your body recover.