It sounds like watery Diarrhea ,I also think a check up would be a good thing, make sure she isnt dehydrated .Good Luck
It could be a few things. Your granddaughter could be having gas and some fecal matter is passed at that time - then mixing with the urine in her diaper. Or, your granddaughter could be very dehydrated - that can cause urine to take on a brownish hue. Or, your granddaughter could be having diarrhea and it is coming out in small amounts between diaper changes - that can be common when a baby is having a hard time passing a bowel movement causing the diarrhea to be followed by perfectly normal stools.
The dehydration issue would be very serious. The baby's mother needs to talk to her pediatrician about what is going on.
sorry that was posted to the wrong section