The blower was working when the heat was not. Also, I believe the same circut runs the AC which has always worked.

This could be many things causing this. You need to add more details as to what is going on.
Does the blower work? In other words, when you turn the heat on do you feel air blowing out of the vents? If not, then there could be a problem with your blower not blowing the warmed air out. This could be anything from something as simple as a bad fuse, a stuck vent-door to something more serious like a burned out blower motor.

If the blower is working, it might be something to do with the thermostat. This is generally located in a small housing at the end of one of the water/heater hoses running to the engine. On most cars this is very basic, cheap and easy to replace and repair, but the way things are being built today you never know what you are going to run into on some of these newer cars.

I would start by checking the blower motor fuse, thats the easiest and cheapest possibility.

You don't have enough coolant in the system. When the air pocket gets to the heater core, it stops blowing hot air. When the air pocket leaves the core, you get heat.