5 year old is acting inappropriate at babysitters.?

I have a 5 year old stepdaughter that right now we are sharing custody for the summer. She is normally good around adults and very well mannered. She goes to the baby sitters and plays with other kids. Lately she was caught stripping. She shows her to other children or just pulls down her pants. We have tried talking to her about why she does it and of course she says "she does not know"...me and my husband are completely frustrated...it doesn't happen that often and we do punish her for her inappropriate behavior by giving her quiet time or time outs and lectures about what is appropriate and what is not. She is 5 and I feel she should know better. What should I do? Should I be concerned? is this normal? I want her to act appropriate with other kids so I don't have to worry.