An againclothesroom 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.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
I’m in the dressing room looking for her locker now.