Why am I gaining so much weight?

So I just made the track team and everyday I run at least 2-3miles. I do minor ab workouts aswell. When I get home I am soooo hungry and i figure I can afford the extra calories so i eat. However, I have been eating ALOT. Today I weighed myself and I gained 3 pounds. What is it? muscle or fat? Am I eating too much. How much do I eat to perform well without overeating?? Why do I feel like I’m gaining weight?