Hello and welcome to healthcare forum.
Function of ventriculo peritoneal shunt is to drain cerebro spinal fluid to peritoneal cavity
As you said the shunt is functioning normally, it happens sometimes that the fluid is drained to peritoneal cavity by the shunt properly but not absorbed from there, which is collected there.
This collected fluid produces pressure over the stomach producing pain, vomiting
and also abdominal distension.
The condition can be diagnosed with abdominal u. s. g.
Treatment includes u. s. g. guided aspiration of fluid.
Best of luck.