It was Cecily's understanding that the mess on the girls' robes was tied to that particular lavatory. That could explain Miss Fudge's reaction about it, as well as the prefect pointing at her robes.

She still couldn't understand just what was going on. Was the lavatory cursed? Was a student there throwing dirt on people? Why couldn't they speak?

Cee pointed at the lavatory and then twisted her wrist in a gesture meant to convey something along the lines of "What's going on there?" With her other hand, she took her wand out of her pocket.