That is probably the case. You can run out to WalMart (or a local auto parts store) and get a recharge kit (get one with a pressure gauge). Just follow the instructions. If the pressure gauge shows proper pressure (either before or after adding refrigerant) and it still blows warm/hot air, you have a problem with something else.

 

if its blowing warm air then you got 1 of two major things going on (there hundresds of things that could go wrong but these are the most simple and commen) 1) and this being the most commen your a/c is out of refrigerant which would in the end result would end up being continues warm air. You can go to walmart like stated above and buy the recharge kits simple clean easy. Be sure to no over charge or under charge the system or you will have the same result. Also you will probely wana take it into a shop so they can find the leak as depending on how big the leak is it will do it again maybe in a week maybe in a year (its them dam rubber houses they leak so easy) or if you wana you can get soapy water and spary it on all the connetions and look for bubbles (after you recharge the system of cource) then 1s ya find it fix it. 2) The compresor is shot or in other words shes not compresing the cold vapor refrigerant into a hot vapor. Turn the ac on full blast pop the hood and listen the compresor see if it makes any nocking noises or other akward sounds. Im not to farmilar with car a/c so i dont no what kind of power shes post to be pulling or amps but a local shop can do a efficenty test for you for probely 60 bucks at a good shop. but id say you out of refrigerant from what it sounds like.