My girl is very thin, as, I am interesting why does thin to happen?

Childrens on a same conditions developing are different, they are for long time keeping thin, other are raising weight,and in thin teenagers appetite is bad, as causing stress, nervousness, irritability,bad sleep, talking in sleep, nausea feeling, ache in belly, within all day aches repeating, as in over loading  nervouse system, organs are still developing, were more time watching tv, doing lessons,or play in games,as causing fatigue, nervousness,appetite bad, and on a holiday,when load is poor appetite will improve, and better happening a digestion process and development organism, as therefore when nerves as bad,they need to cure,with remedies, physical effects,on a light cases helps vitamins, as vitamin c uses for three times/day before meal hour.

And other reasons for thin are tonsillitis, rheumatism, tuberculosis, and parasites,and genetical, if mother or father had been a thin.