Q: Heater and Coolant Leak

I first noticed a small spot under my car in my driveway last spring, but it only happened a few times so I did not think much more of it. Now my heater is not functioning properly. It will kick out cold air at times with the heater on high and engine temp right in the middle of the gauge. Other times, mostly on highway driving or when I gun it, then the air will get hot again. The temp gauge in the car only creeped up twice. Once when sitting in a drive thru line and other time driving 55mph (but went down when I turned the heat/cold air on). I topped off the coolant (it was VERY low), and it appears in a few days the coolant level in the reservoir has already dropped some. Therefore, I assume there is a coolant leak somewhere. I would rather not jump to the conclusion to change the water pump due to the HIGH replacement cost, so are there some other things I can check first?

My car has 126000 miles.
My car has an automatic transmission.