That means the coolant has leaked into the oil system. That is not good. You might have a blown head gasket or something like that. Get it fixed ASAP, or the sludge (mixed water & oil) will damage the bearings inside of your engine.
Sorry about that.
2007 BMW 335i Sedan, Montego Blue with Lemon seats, AluFelgen CS7 rims & Continental DW tires