She remembers what happened before her clothes were stolen...

Myrtle! That ghostly bitch was blackmailing Hermione in the bath. She probably took the clothes herself and hid them down in the second-floor girls' bathroom to try to coax Hermione there.

With newfound hope for getting her clothes back, Hermione practically hurtles down the stairs of Gryffindor Tower, ignoring the Fat Lady's instructions.