If you're certain the compressor is running, then you probably have a leaky monovalve. The monovavle controls the flow of hot water into the heater core. If it leaks, you are in effect reheating your air conditioned air.
Some models have rebuild kits available for the monovalve, but this is not true on the 124. However, some folks have had success just dissassembling the valve, cleaning it up with a kitchen pad, and reassembling.
As for identification and location of the monovalve I refer you to the search feature of the bulletin board - this topic gets discussed to death every spring. Good luck.