Will i gain weight while being sick?

I really need help because im very scard to gain even a half of a pound. Im 12 year old teen girl im am about 83-84 lbs and 5.1 tall. I do sports 5-6 times a week (3 times tennis and 2 or gym), i also have sport in school 2 times a week but i dont rly participate. I always eat about 80 calorie lowfat yoghurt in the morning and often skip lunch so i burn all my calories off. But today morning i got sick so i spent all my day on the sofa and almost didnt move at all. My mom made me eat a seafood paella for lunch, i also ate some fruits but today 4 dinner my parents brought me my favorie veggie middle size thin pizza and i ate it all. Now im really afraid to gain weight because i might stay some more days at home and my mom makes me eat much more food than i usually do. Will i gain weight and what should i do so that it will not happen??