Could be refrigerant, could also be almost anything else. If you download the service manuals from this site and follow the procedures, you should be able to figure it out.
I have a similar problem, but it's not right when I turn it on. I can operate the AC for about 10 minutes or so and then it starts blinking and causes my car to surge (revs bounce up and down) until I hit the AC button to turn it off. Then, I can immediately turn it back on and it's good for another 10 minutes. I have a feeling it has something to do with my compressor going out but I haven't really messed around with it.
In the service manual it lists about 6 things that could cause the AC light blinking, I just forget what they all are.