Why am I always bloaty??? ?

I am a 13 year old girl... And I always seem to have trouble with being bloaty. I know I am not fat, at all... And everyone tells me I have a perfect weight. When I wake up in the morning my stomach is completly flat... But when I eat my stomach seems to almost grow through out the day... I don't know of it is because I'm not getting enough water or...??? Also, I am very active. I play club soccer, golf and tennis multiple times every week. Also my mom has a raw diet, so I'm eating very very healthy and deffinatly getting my full servings of fruits and veggies. If you have any suggestions on how to help with this, it would be really appreciated. Thanks!