Are her pupils yellow? Insides of her ears? If so, it could be liver issues.
If its just the outside of her mouth, she may have chewed on her bedding or eaten her poo.
She could have even thrown up bile from an upset stomach. Bile is yellow.
If she's eating, pooping, peeing and has a normal energy level then watch her closely for a few days.
If she's not eating, pooping, or peeing normally...then vet time.