Heater not working properly. 1999 Intrepid

The heater is not working seemingly at random times.
Take this morning for instance, It was cold outside so I went out and started the car to warm up for 10 minutes or so before leaving. When I went out to the car it was just as cold as before I started it and blowing cold air. I got in and started to work, about 1 mile after getting on the freeway the heater kicked on and started blowing heat. The car never used to do this, I could start 5 minutes before leaving and it would be toasty inside by the time I left.
The other day this same happened with my girlfriend in the car and she suggested that it was because the gas was low (haha). So I obliged and filled up "knowing" that couldn't be the problem, but after I filled it up the heater kicked on and she was like, "I told ya." I'm still not convinced that has anything to do with it but she now thinks she's a certified freakin mechanic!

Anyone have any suggestions? Its cold and I have a 1 year old, so I would like to get this fixed ASAP!

Simple solutions first: check coolant level, check for kinked hoses, tightly capped radiator......
NOw...randomly heating/non heating heater can be caused by exhaust intrusion in the cooling (heating) system from blown head gasket...but this comes with other symptoms, like steaming, overheating of engine when you freeze inside the car..haha..no, not funny...but I just wish these symptoms do not prove themselves.