Is there a bubble in system or replace heater core

Since you started having the problem when you replaced the water pump I'd suspect air is trapped in the cooling system. You'll need to locate and open the bleeder screw(s) with the engine cold. Add coolant until you get a steady stream of coolant from the bleeder screws. It's helpful to park the vehicle on an incline with the front of the vehicle pointing upward, like halfway in the driveway. This way the heater core is the lowest point of the cooling system. Hope that helps! Jim