Sounds like you should return to the shop that looked at your car. One sign of a good shop is one that honors a 12 month 12,000 mile warranty on their work. Sounds to me like they MAY have misdiagnosed your car. A good practice of any shop who works on a/c is to put dye into any system they work on. I am willing to bet there is a leak in the system. And, why did the tech say the system needed to be flushed? Did they find contamination in the system?