Most A/C systems leak a little every year starting when new. It is possible it finally hit the point where the low pressure cut off sw. said 'thats enough'.
To do air right you should have the tools. If you dont want to be bothered with getting the right tools, Larry's thread is the way to go. I have enough cars that have A/C to justify me owning guages, a vacuum pump, electronic leak detector (sniffer) and a dye kit.

I like the idea of performing a vacuum test first before doing a sniff test but I like the sniff test before the dye test. Pull a vacuum down to -30 on your system shut the valves off. If it stayes there, its not leaking so charge it up. If it slowly looses vacuum over a peroid of 1min., inject the system with a little R134 and do a sniff test. If you can't find the leak, do a dye test. When ever the system is opened up, you need to place baggies with rubber bands over the opened A/C lines to keep moisture from reaching the dryer and dirt from clogging the expansion valve. Moisture in the system expires the absorbtion of the charcoal pellets in the dryer.

You can always pay a mechanic to do a vacuum/leak test for you, then replace the parts yourself.

Note! when jump starting the compressor you will need to disconnect two connectors on the low pressure cut off sw. Be careful when disconnecting these because they tend to break off after they get older. I always squirt these with WD40 a while before I expect to disconnect them.