You have air in the system. I had the same problem the day I drove my car home after I purchased it from the origanal owner. All I did was add coolant to a COOL radiator, start the car, open bleeder valve, the bleeder valve should hiss a little meaning the air is ecaping, when water starts to come out close the valve. That is all I did and it has been fine ever since. I don't know how it got low to begin with because it has not lost any coolant since I added it over 4 thousand miles ago.

Dean