is a condition that can be due to various reasons like poor eating habits, untimely meals, disturbed sleep, stress and disturbance in digestion. As you mentioned you don't feel hungry that can be the sign that there is some issue with digestion. So first try to improve your digestion so that nutrient absorption will increase. For hemoglobin you need to check with doctor as it is quite low and you are feeling weak as well.
In mean time you can start eating nutritious meal as it will definitely help reduce your weakness. Following things will be helpful for you for weight gain: 1)eat frequent meals (light meals if have digestion issues) 2)add sufficient proteins like egg, dal,milk, curd
3) drink enough water
4) avoid lot of junk and fried and sugar
as it gives unwnated weight gain
5) add more of vegetables and fruits in diet
6) include at least one portion of nuts daily like almonds
, walnuts,dates, anjeer
7) have enough sleep daily.