Does the coolant level in the radiator keep getting low also? When that happens, and your reservoir tank keeps filling up like yours is, you have a bad head gasket.
if your car is running hot it may also boil which will make it expand and some will be forced into the overflow. are your coolant temps consistent? the headgasket thing seems highly likely but you would notice your car running shitty if that was the culprit. so how does your car run? is it running just fine and strong as ever?
