Feeling very weak, tired, breathless with a pale skin and appearing almost like a person who is anemic, are the symptoms of leukaemia. It would be more appropriate to have you undergo medical check-up and then only arrive at a conclusion. But the overall indications point towards Leukaemia – the cancer of white blood cells. If not treated promptly, it could lead to acute Leukaemia wherein the condition progresses rapidly and aggressively and requires immediate treatment. The description provided by you point towards acute Lymphoblastic Leukaemia.