It's not good if it's warm because the light pulls more power at start up so it can be a fire hazard. Easy way to fix is buy a cord rated for 15/20 amps. Make sure you have it pluged into a GFI.
The heat essentially means that the cord cannot carry sufficiently the amount of current that the heater is asking for. It will do it anyway of course, but the result is the excess heat you feel coming from the cord. Eventually that heat will break down the insulation around the wiring, resulting in even more heat build-up and potentially a fire.
Extension cords have amperage ratings, though they are often not obvious. You should ALWAYS make sure that the cord used is enough to handle the job, by comparing it's rating to the current draw of the item being plugged into it. If more than one item is plugged into it the amps from each device should be added together to determine the total capacity needed.