Originally Posted by irt
There is a vent under the shower, but the little air that comes out of it is pretty much cold
If air is coming out and it is not warm but the air out of the other registers is warm then yours may not be a simple problem. You will need to take the register out.
I suspect what may have happened is that the duct is not connected to the register but is blowing the hot air in the space behind the register so that what actually comes out (a) is reduced in volume, and (b) had dropped in temperature.