What causes painful stomach cramp during dialyis?

Stomach cramping. Rapid fluid shifts, electrolyte disturbances, ? Uremia. But make sure to mention to the doctor and the nurse.
Dialysis. This is usually caused by your "dry weight' being set too low. Alternatively you may have excessive weight gain between dialysis sessions. The other cause is a drop in blood pressure during dialysis. This can be easily corrected by changing the settings on the dialysis machine and controlling fluid intake between dialysis treatments. Uremia only causes this during the very first treatment.