Muscles loss and I am look like child.

Currently my weight is 37 kg but 3 years ago I was also 37 kg. I am 18 years old and my height is 5 feet. In these three years my body muscles are decreasing, for these three years my belly fat is also frozen, but in these three years my body muscles have decreased, but my belly fat has not decreased  And at this time I have become very thin, earlier I used to feel very hungry but slowly my appetite also decreased and now for four to five months I am not feeling hungry at all, I am not able to understand how. Is a disease because my body is getting thinner by itself but my belly fat is the same and my weight is not decreasing as well. Please tell me what kind of disease I have and what kind of doctor I should see. Can I be able to grow after 18 because I look like 12 years old and my chest size is only 25 inch and and my waist size is 28 inch because of stomach fat.Suddenly my body hair is also grow very large size in 1 inch