Eat healthy and think positive.. try taking milk and banana in mornings.. check your
thyroid, hyperthyroidism causes weight loss.
Do exercises and jogging it'll help your muscles to vain healthy weight.
Moreover stay happy and stop over thinking about this, as it will merely make u sad without much results.
Your metabolism might be high, so whatever u eat gets digested fast.
Next Steps
eat dry fruits, banana milk and honey milkshake, eggs, mayonnaise, butter. but in a limited quantity. don't keep having junk just to gain weight.
Health Tips
stop stressing amd stay happy. meet a good dietician and follow the advices.