Few Questions:

1. Are you in stress?
2. Is it in your family - i.e. members normally thin ?

If not, get checked with doctors, and confirm that you are not suffering with any diseases.

So far eating stuffs, eat something like banana, milk (normal), and anything that you love eating (except junk food, alcohol). Dont eat excessive at one time, instead eat normal amount at regular amount.

Also, I suggest you to take vitamin B6 capsules (or take B-complex tabs). This helps food absorbed.

Last but not least, try to sleep in time, and sleep enough (at least 8 hours) in 24 hours.