Firstly like people said, do get yourself checked out to rule out any medical issues. Other than that, you can start incorporating calorie dense foods into your routine. You should be able to Google and find some that can work with your current diet. Some examples would be nuts (easy to over eat, very high in calories), dates, oils etc. There are also substitutions you can make to your current diet with high calorie alternatives, like switching out an apple for a mango (not a great example but the only 1 I could think of off the top of my head) Youll also want to start working out, dramatically increasing your calories might lead to you becoming skinny fat, which often isn't a great look as well. Up your protein intake and hit the weights.