First look at the temperature gauge on the dash when the engine is warmed up. If it's lower than normal, suspect a defective thermostat that is opening too soon and preventing the engine from reaching normal temperature.
If the gauge reads normal, feel the two heater hoses after the engine is warmed up and " heat" is selected. If they feel cool, suspect a plugged heater core. This used to be common on older Caravans. Sediment would collect in the heater core because it sits so much higher than the engine. I've had very good luck on those older models with removing the heater hoses and flushing the heater core with a garden hose.
If the heater hoses are hot, there is a problem with the electronic controller, a broken door in the heater box, or a problem with the vacuum hose or motors that run the doors. Based on your dandy observation that it blew warm air for a while, I'm leaning toward a plugged heater core that was partially plugged earlier. Actuators and temperature doors in the heater box generally fail suddenly, and all at once.
If the problem gets worse, to the point of engine overheating, suspect a worn / loose / broken impeller on the water pump. I haven't heard of this happening on Chrysler products but it's very common on Volkswagens, so it's worth mentioning.