She had to pee

Before she went out for the bath, Hermione had had dinner and with it had drunk lots of pumpkin juice. As she has prepared, she began to notice a slight urge to urinate. However; she was so eager to enjoy the soothing waters, she figured she could wait until after she was done to go. The circumstances which led to her current situation had distracted her from this need, which had grown over the course of the evening. It wasn't until she was outside the Prefects bath that she felt the need again, stronger this time. Uncharacteristically cursing herself for not going before, Hermione turned to go back inside when she remembered that she couldn't without her prefect batch and it was still attached to her robe which had been taken earlier.

Assessing the situation, Hermione was left with two choices. One, as she was out of the dormitories past hours and Mr. Filch and his cat would most likely be roaming the hallways, she should try to reach Gryffindor Tower as quickly and discretely as possible with having her nakedness exposed. Two, though her urge to pee wasn't too strong, it would undeniably grow the longer she delayed. She wasn't sure if she could hold it long enough to make it to her dorm. She should try to find a bathroom to use before proceeding further. But doing so would increase the chances of someone seeing her naked.