Electrical problem

Could be your alternator is getting weak, An alternator does not necessarily fail suddenly, but can gradually lose it's ability to produce what it should, and when you put the load on it with the A/C the system detects that voltage drop. Hence the light. Any other lights come on?

I use an Icarsoft i906 to diagnose precisely what is wrong, especially when it comes to A/C and sensors. Eg it picked that my left rear parking sensor was the one not working. And on a friend's Saab it picked which A/C flaps were stuck.