Darren,
I would suspect that your thermostat is sticking shut. When your thermostat is closed, your coolent only travels to part of your engine. Until that thermostat starts to open, no hot coolent will enter your heater core; hence no hot air coming out of your vents.
If you see your temperature return to normal, that means your thermostat opened up and started circulating hot coolent through your heater core, and that's when you feel the warm air coming from your vents. Having that thermostat replaced is a fairly quick, easy, and inexpensive fix.
If replacing the thermostat doesn't fix your problem, you may have a faulty water pump. That is not cheap to replace. Have a qualified mechanic inspect that for you.
Good luck!