When was the last time you checked your coolant? Have you been using your car for a lot of short trips recently? Short trips do not enable the engine to get warm enough to get rid of the water condensation and this may result in some creamy deposit in the coolant tank. This is particularly true when the weather is cold.
The worse case scenario would be a blown head gasket. In such a case the coolant level would get low quickly and you would see white smoke from the exhaust. In any case you need to flush and replace your coolant and then look for sign of head gasket after that.