Nuts can be a great source of calories! And they are monunsaturated fat, so not only are you getting calories, you are getting healthy one.
Seeds can be great calories too, sunflower seeds etc.
Maybe try getting a bag of trail mix or a mixed container of nuts.
Carbohydrates can also be helpful in gaining weight (personal experience). But its usually the bad carbs....not sure if that can be done with your allergies.
A key to losing weight is cutting calories...so the opposite is true. Use fitday to track your calories, and eat MORE than you burn. So if you work out at all, then you have to eat more.
Last thing, muscle weighs more than fat, so working out isn't a bad thing....but you are burning more calories, so it's a toss up.
Good Luck! And welcome to fitday