Hi, Welcome to Health care magic forum.
The B.P. is unstable means there must be some problem in the cardiac system, or there may be acute
anemia, or problem with the absorption of food taken.
The other reasons may be some chronic infections like
tuberculosis etc.
To gain weight take more of calories, fats, and oily foods.
Sleep for longer time, practice early to bed and late to rise.
Take more of green leafy vegetables, pulses, sprouts, and protein rich foods, to have good recovery, and resistance against infections.
Wishing for a quick and complete recovery. Thank you.