2002 Chevy Cavalier Coolant light flashed

I had a new engine installed at 98000 miles. Everything has been fine. Last week, I went into the dealership for a routine oil change, and they told me that my water pump was bad. I had my water pump replaced, but 3 days later, I returned because I smelled anti-freeze when I would turn on the heat. The dealership told me that they checked it, and it was fine. The smell was probably due to spillage, and that it should go away in a few days. Everything has been ok, until today. I was ideling on a steep hill, and my coolant light came on, and it remained on for 2 or 3 minutes, and turned off. My coolant level is full, so I don't know what to do? What could it be?