Why does this happen?

Okay, so i have athletics 5th period. i only have 5 periods a day, so its my last cl of the day. usually we have to run a mile or 2 every day. lately, i've been experiencing liquid like shaking up in my stomach when i'm running. like you know how water bounces up and down when you run? that's what i'm talking about. my friend says its because i'm not drinking enough water, but i'm thinking that's the source of the water jiggling. any help please running experts?