Did you crack the bleeder valve and let the air out of the system after you replaced the water pump? If not, you probably have air trapped in the system. Could be your heater core leaking. Check the carpet on the passengers side front where the evaporator and heater core are housed. If that's not it then the only other place it could go without being visible is through the cooling jackets and into the crankcase which would mean a blown head gasket. Do you have steam coming out the exhaust or milky oil?