Ehh, a thermostat stuck open, unless you're in an extremely cold location, would still allow a vehicle to warm up enough to get some level of warm air. Plus it worked for a few days then the issue returned after the flush.
You didn't say if your temp gauge is showing normal when these conditions exist but it sounds more like an air bubble in the cooling system. You said your coolant level was low so you got it flushed. You should be more interested in finding the source of the leak. Fixing that will prevent coolant from escaping and air finding its way back into the system (which is probably why your symptoms returned). If you want to test this out for free make sure your fluid is topped off, drive the pathy up on a set of ramps or somehow elevate the front end, open the radiator cap and let it run. You may need to rev it a bit to get the fluid moving to flush that bubble out. Replace the cap, get it back down to ground level and check it out.