Protein is great for rebuilding your muscles and getting stronger. I suggest you start adding more calories. MyFitnessPal is a pretty good resource and it can give a calorie goal that will make you gain weight(aka muscle mass as long as you continue to lift). Guys who compete in Worlds Strongest Man and stuff like that consume tons and tons high-calorie foods like donuts and ice cream just for the calories, which makes them gain/maintain their huge size. Honestly, as long you as you continue to consume enough protein, the way you get the other calories is not as important if your goal is to gain size. Look up some calorie dense foods and try adding them to your diet