1991 Subaru Legacy overheats when heater is on

My car has always run great. A while back I ran the heater full blast for a while for the first time in a long time. It caused my car to overheat. My coolant was very dirty and hadn't been changed in a while. So I flushed my radiator and put new coolant in. After that my car overheated anytime I drove it. So I had a professional mechanic replace the thermostat. It seemed fixed for a while, till I turned the heater on. Now my car heats up to running temperature in less than a mile, and overheats on longer drives (40 minutes or more) and overheats when the heater is turned on. So I have just been avoiding turning the heater on and keeping my drives short, I turned the heater on yesterday for a short time to see what it would do and it blew hot at first then started only blowing cold. My coolant levels are good, I check it often, pretty sure theres no air in the system. Seems very strange! Any ideas? No one believes me when I tell them that it overheats with the heater on!