His mind was flooded with new memories. Both he and Karyn were in an all girl group with Xtina. He played the drums, something he was good at, but never found a group to join. Apparently, the reason he dressed like her was part of the group image. The group was the Xtina's, so they all dressed like Xtina for concerts.
Xtina needed a drummer after the last one bailed, but had a thing against guys being in the group. Karyn wanted him in the group so bad, Xtina finally gave in, but with one condition. She figured it would break Jon if he had to dress and act like the girls on stage, and to prove he could do it, he would do it nonstop for a month before their first gig. So, that was why Jon was in her clothes.
As for the meeting, it was for practice, and for Xtina to verify he was still dressed appropriately. She was going to make his life hell for the month, and he was stuck!