In the end of "Picture This!", why does Mandy leave?

Mandy thought that the reason drew went into the bathroom was to start the shower but he washed his hands. The reason she didnt want the shower started is becaus his brother got the girls to get in the shower (i think) with him. Mandy thought she was one of the girls who he tried to get in the shower but Drew really just wanted to spend time with her and he was just washing his hands because they were sweaty.