Mewn

The answer is overeating. Eating more calories than your body uses leads to storing it as extra energy in the form of fat. Doing this even a little, for a long time, adds up. Think of it as putting a dollar in a piggy bank every week or two. Ten years later you'll have quite the amount of money saved up.

Have your allergies made weight hard to control? How tall are you/how much do you weigh?

I have a friend who is "allergic to everything" it seems, and he's skin and bones due to the few things he can eat tend to be low calorie, mostly.