Protein is more important than calorie intake, but calorie intake is still EXTREMELY important. If the aim is to build muscle, you're still going to need calories to burn for energy, especially carbs.
As someone who struggles with weight due to an illness (check my profile and posts), my biggest advice is this -
It's going to be a struggle, but you have to force yourself to eat more. 500-800 calories is normally what you should be intaking in one meal for weight gain (as well as the right amount of protein for muscle gain).
OP, it may hurt your stomach, but you have to try expand it slowly and get your body used to taking in more food. This will take a while considering you're used to a low intake.
Exercise will also help make you hungrier. You'll realise the more active you become, the more your body is going to demand you provide it energy.
Good luck!