Check your coolant level. Had a water pump start leaking on our 1480 a couple years ago and it dropped the coolant level enough that we weren't getting heat in the cab....that's how we found the leaky water pump before any damage occurred.
I don't want to make you feel like an idiot or anything but it happened to my dad just yesterday, were the heater valves maybe on previously and you turned them off by mistake thinking they were off, or maybe just one was off and you turned one off and the other back on. I'm just sayin stuff like this happens to the best of us weather we like to admit it or not. Disrequard this if you double checked