1997 Plymouth Breeze No heat and coolant leak

HI folks,
I am driving a 1997 plymouth breeze. It has been slowly leaking coolant for some time. I put it in a local garage after the heater stopped working and the coolant loss increased. They replaced the thermostat and refilled the coolant, that was ten days ago. Today I topped up the coolant again after the heater stopped blowing warm air again. No change it the heater. I searched the internet for similar problems other people were having and noticed some of the answers. I just drove around the block with the heater/fan on bust, no heat. I engaged the A/C button and within 30 second warm air was coming out of the vents.
Any ideas as to what my problem may be?