Dehydration can cause stomach cramps and kidney problems.
Dehydrations means you don't have enough fluids in your body (as probably you all know) but your body still wants to get fluids in its blood.
1.The way you get fluids in your blood is by getting it out of you food and fluids that is in your stomach/abdomen. Your body urges for fluids so takes like everything it can out of your abdomen. The feces that's left in your abdomen can get really hard and that is causing pain
2. If you have bad diarrhea it can cause dehydration. Diarrhea can cause stomach pains.
Then indeed there are the kidney problems when dehydrated. It can cause pain aswell.. but not in the stomach, but in your back (at the place of the kidneys

).
Then there is also the possibility of miscarriage. But most of the time these women will stay in the hospital for observation. Britney didn't stay so then I'd say they made a scan, saw it looked alright and send her home.