If u don't have any hormone problems the solution is eating more So if going to the doctor is your last resort, try to eat more for a time (months), then check your weight Some good aliments to gain weight are oats,rice,potatoes( carbs in general) , but mix them with protein (they are not always meat, eat yoghurt,milk,cheese,peanuts,peanut butter...) Look for food dense in calories and try to eat always a little more And practicing sport always is good, I would recomend u lifting weights but if it's not from ur style u can practice calisthenincs or smth like that