Your coolant level is dangerously low. It's only circulating enough to get to the heater core when the car is generating more than idle RPM. At this point, purchase at least a gallon of the "mixes with all types" of antifreeze and have it handy at home when you are ready to start.
The best thing to do with the gallon of antifreeze is to at least have in in the house to get to a warm room temperature unless it's summer out and hot. Let the car warm up with the heat on full and A/C off, and while it is idling and LEAVE it idling the whole time, be VERY VERY careful and slowly click the radiator cap to the first stop. Wear leather gloves and use a rag because it may push some very hot fluid out. Once you have gotten it to that click and it has released pressure (squeeze your upper radiator hose to determine that it's soft), then remove the cap the rest of the way. Then, slowly, pour antifreeze into the radiator until you see it coming up to the neck. Again, be VERY VERY careful because there is a possibility that it could force fluid back out at you in a geyser. Add antifreeze slowly until you can see it about 1" down from the top and then check to see if there is heat inside. Let it idle for a few more minutes and check again. You may see that it has sucked it down in. Add more antifreeze, again, slowly, repeating this until it stays up near the top.
Put your cap back on and then let it continue to idle until you see that the top hose has tightened up with pressure. If it doesn't get tight, then you most likely have a leak and that's why you lost the coolant in the first place.
Good luck and BE CAREFUL. Hot coolant really is all it's cracked up to be....DAMN HOT.