I have an Amana furnace and for some reason the cool air keeps blowing out. Both the furnace and the ac unit seem to be running at the same time. Earlier today I was able to get hot air out of the vents, but now nothing but cold air. The thermostat is set to heat, so I don't know why the ac unit would run, any suggestions?

There could be a short in the thermostat wiring. The thermostat could also be defective. I am sure there are other items that can cause this also but I would check out these first.

While you are getting the cool air out of the furnace make sure that the burner on the furnace is burning. It could be that you have a bad connection and the burner does not come on but when the time comes for the fan it will come on by a timed cycle.

Did you do any work to the system or change out the thermostat before this happen? If not I agree with hvac1000 you may have a short in the low voltage wiring.