Make certain that your heater hoses are attached to the appropriate connections on the engine. Generally, the supply to the heater core is the larger of the two and comes off either the intake manifold (next to the thermostat housing/water outlet) or directly off the water pump. The return (smaller one) dumps back into the system at the top of the radiator.
They can't both be on the engine (i.e. one on the water pump and the other on the intake manifold.) if they are there will be no flow. If you are piped up like that, it could also explain your temp gauge spike.