Pull the thermostat out and put it in some boiling water on the stove. If you don't have a thermometer that reads high enough, watch for the little bubbles and see if it stays shut or open even to the point of boiling.
If your gauge is wrong (temp) see if your radiator hoses are getting warm. If they are, then there may be a valve that controls water to the heater core. If no valve, your heater core may be plugged up. I recommend a flush and then refill with new anti-freeze.
I really do help you find out what it is. When you do, please post it just in case someone else runs into this problem.