Changing Room
Locker Room, Dressing Room
An again clothes room is a changing room. You get to change again, even after changing in the morning for the first time to leave your house. This can also be a locker room or a dressing room, so really anywhere you can change that's not your home, usually.
こういしつ
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
今更衣室であの女のロッカーを探しているところよ。
I’m in the dressing room looking for her locker now.