Weight gaining issue. Or thyroid?.

I have thyroid and ulcers but not in a extreme condition. But the problem is i'am not gaining weight. My current weight is 37kgs but when i get sick like suffering from fever or something then my weight drops to 35kgs easily. Iam not able to gain weight with all my efforts. Hardly I can manage to get 38kgs. Also I have fever at night, not continuosly but 2 days a week probably and only at night. I feel very low sometimes. I tried exercising at home,eating food according to fixed diet plan but it shows no response. I want to find out why it is happening. Is it because of the ulcers or thyroid?. But i take medications for them. Its in a controlled condition. I feel like eating, I feel hungry but when I eat though my stomach is not completely full i get a feeling of fullness. And I dont get enough energy from it. What would be the better treatment for my condition.. Please help me out. Thank you