Brando

All of a sudden my heater started blowing cold. I felt the hoses around the heater valve and the ones on the heater core. The inlet hose to the heater core is hot all the way down. The return hose is cold all the way up. I flushed and refilled the entire system and it seems about the same. I did notice that it puts out good heat with the ac button turned off. But when I turn it back on it gets cold again. These were both with the heater set all the way to HOT. I just had the thermostat and water pump replaced. Both of the radiator hoses are hot to the touch. I made sure the system is completely full of fluid. Any ideas what to check next?