1998 Chevy Truck Slowly loses antifreeze but now?

I have a 98 S10 and recently replaced the thermostat when I noticed the temp guage was dipping lower to 100 degrees and blowing cold air. I filled up with antifreeze, but then a week or 2 later there was a loss of antifreeze and it started blowing cold air again with the temp guage dipping back to 100 degrees. Now, after filling it up with antifreeze again, it heated up but then dipped down but still held fluid. I can't get anyone around here to tell me what it could be. Any ideas?