Blood counts dropping even on chemo break..why??

My mother was given an extra week off between treatment (AC chemo) to give her counts time to get better. They were borderline last week but since she was stable at last scan the onc decided to let her have an extra between week infusions from here on to get her counts up. Today her numbers had dropped even lower and was told she needed a blood transfusion (2 units). Last cycle she was given an extra week as well but during that time her counts went up considerably. The onc isn't sure why this is happening. They did an echocardiogram to check her heart and it was fine. She is already scheduled for a routine heart MRI at the end of the month because of being on chemo so long as well as a routine CT scan.

She is not bleeding from anywhere (no blood in urine or stool), she feels fine and not unusually tired or anything. Heck-- she even looks good with no paleness. Has anyone had this happen?