hi i had gone through your query and understand your concerns. i would come up with the possibility of ALLERGY
is the underlying for all your sufferings including underweight
-usually the energy came from food absorbed from the intestine into the blood.the blood contain oxygen
( oxygen acts as a vehicle to the Hb)which carries the energy throughout the body.
-if a person suffering with allergy has less amount of oxygen in the blood which cant be sufficient to distribute the energy leads to malabsorption
and is the cause for your underweight.
i advise you taking Homeopathic
medicines gives safe and permanent cure for your complaints
consult your local homeopathic physician for correct diagnosis of the case and remedy to fit your complaints
I hope this is helpful for you, thank you