Vampire Diaries: Question about Katherine/Klaus compulsion?

Because Klaus had compelled her to stay in the room before she took the vervain she was still bound by that compulsion until he released her. Although she could not be compelled to do anything new while on the vervain she was still bound to any compulsions which may have occurred before she began taking it. That is why she was not able to leave until the most recent episode when Klaus told her to.