The heater WAS on a thermostat. It kept tripping the circuit, so when we had a spate of really low temps I plugged the heater directly into the wall instead, to be certain the heater wouldn't get tripped and not have heat in the room.
So it didn't stop heating... and just heated the room really super hot. It should have gotten to about.. oh.. 90? But for whatever reason.. it just kept heating.
It's my own fault, I should not have plugged it in, but I DID run it that way all LAST winter, with no issues. I'm just so mad and upset with myself.