Slow coolant leak

I have a 2003 Pontiac Grand Am with a V-6 engine and 73,000 miles, the low coolant warning light came on about 1600 miles or so ago and it was time for an oil change so I took it to the dealer and they changed the oil and topped off the coolant stating it was just a "hair" low and probably nothing to worry about it and they didn't see a leak. I had to add coolant again about 1400 miles because the light came on. Both times It doesn't stay on, it comes on briefly and then goes off. The temperature gage is staying where it normally stays. Any thoughts. I really can't afford a repair bill and I still have about 10 payments left on this car.