It's probably the Pre-K giving her grief. They have to staff so many teachers to keep the right teacher-child ratios. When parents take children out, they expect to not have to pay, but what is the center supposed to do with those teachers? It's a tough thing, but most day care's only allow so many absences, changes to the schedule, and then you have to start paying for those changes.
I see. It's been a long time since I had kids in daycare, but I just paid the same as if they were there and kept them out when our family was able to spend time together.