98 Sunfire A/C Problem

Ok, so earlier this year I noticed that my A/C was not blowing cold air anymore. A few weeks ago I took it to the dealer who said that there were no leaks and recharged the freon. Well, now, it hasn't been very long and the stupid A/C is back to only blowing hot air after I spent $100. I just got the oil changed the other day, and it seemed to be working right after that but then quit. Is it possible that they might've done something to it when I got the oil changed? Other ideas?