Hello, My Sister Went In Last Monday To Have A

Hello, my sister went in last Monday to have a heart valve replaced. She was placed on life support for day afterwards, she did fine and was taking off. She was awake and talking and responsive for on the 3rd day. On the 4th day after surgery she has since been unresponsive, only sleeping and not responding to any family, nurses or doctors. CT, MRI and xrays have all came back ok. She ran a fever of 104 and still has 101. Any ideals to what could be happening to her?