Underweight. Hormonal imbalances?

Hello, I'm 25yrs now My weight is 50 kg and height is 5.6 feet  from past 5 yrs I'm unable to increase even a half Kg I'm vegetarian and always take quality foods rich in protein and nutrients and calories If I eat more, only my stomach will become bulk for some time I tried out any diet methods which influence weight gain, but nothing worked Sometimes I feel very weak muscle strength , not able to lift weights and my hands will start shake Is it normal that my weight is constant from more than 5 yrs even some sort of changes in lifestyle Why I can not increase my weight, is this normal or it is treatable It is because of any chance of change in HGH or thyroid hormonal imbalances ? I have very less facial beard and very thin hairs in head and started falling off as well. I'm regular yoga practitioner and no drink/smoke habits, daily I walk min 2 km  Which doctor or specialist I need to reach out to investigate if I have any treatable problems