E.A.
Yup, take her to the vet. Cats are really particular about using their litter box and if they don't, it's usually a medical condition like a urinary tract infection. There was a story one of our vets told us about a guy who had the same problem with his cat and didn't pay attention until the cat climbed up on his head while he was sitting in his lazy boy recliner and peed on his head! That made him pay attention lol.