She could have a UTI or something of the like - a trip to the vet is probably in order.
Other considerations: Did you recently switch your brand of litter? Or has anything changed in your household she would find disruptive?
Chase (Friendly Resident Wigglebum) - Border Collie/Lab/Shepherd X
Kailey (Misunderstood Gentle Beauty) - GSD & foster failure #1
Rupert (Gold-Medal Winner of the 3 a.m. Kitty Destruction Olympics
Heidi - RIP my sweet baby girl