Why i feel so weak?please anybody help me.?

i am male 16 yr severely underweight .my weight is 28 kgs.i dont have any disease.my underweight is going since 4 yrs .also in past i have done anemic profile 1 and it shows low haemoglobin.after 1 yr i dont know why but i feel that i have no energy ,feel very weak .is this due to anemia please please help! Also i wanna tell u that i feel better when i take glucon-d.