I'm no professional, but I'd recommend eating slightly larger portions (adding an extra spoonful of potatoes, or having another slice of meat etc) and look to healthy snacks that are good for energy and sometimes high in calories, e.g. nuts. Don't start binge-eating, or going through lots of unhealthy calorific foods. Keep your nutrition/food types in a good balance. And, as I believe someone else mentioned, consult with a GP or nutritionist if you can, or if any concerns crop up. Good luck and be careful of yourself!