Tige

So not to long ago my head gasket got blown. Apparently the culprit was the thermostat.. so i got the head removed, shaved, got a new gasket and replaced the thermostat... About a month after i noticed my coolant levels had dropped, so i topped it off... A few days after when i went through the drive through at kfc, the car started to idle rough every now and again.. when making my way back home i noticed a deep in power and the car started to behave very sluggish.. when i got back home and parked, i noticed that coolant had mixed in with my oil.... What could really be the root cause of this!!???