94 Chevy Lumina says low coolant but isn't low???

I have a 1994 Chevy Lumina sedan and am having issues. Two days ago the heat gage got into the red. My landlord's dad who was doing some yard work next door and the man helping him looked at my car. It had only a little coolant in it and no water in the radiator and the second fan wasn't kicking on. I don't have any coolant with me so they added water to the coolant reservoir and then they added water to the radiator. After a few hours I drove it around my neighborhood for 20 minutes or so. It would get up to the 3/4 mark on the heat gage and then go back down to the 1/2 mark. I drove it a few miles in town yesterday and it did just fine. I've decided not the run the AC the rest of the summer because that is what they suggested. I got outside today to go to a dr.'s appt and the light came on for low coolant and didn't go off. I turned the car off and turned it back on and it came on again and once again didn't go off. (cont'd below)