it was the end of PE class and since you were late you had to do 5 laps of the sports field without stopping after, so you were the last. After that you were tired and you went back to the changing room, without paying attention that it was the boys' room, since there were no boys left you didn't know that it was the boys' room.

but when you were undressing you heard noise but ignored, the two boys coming out of the showers and it didn't take long before they found you.