2001 Pontaic Grand Am Overheating?

I have a 2001 Pontaic Grand Am with a little over 118,000 miles on it. For the past months now, it has been overheating and running out of antifreeze. I am haven't noticed any severe leakage anywhere and just recently had my radiator flushed and filled by mechanics and they didn't notice anything unusual either. I am running out of options. I keep having to pour more antifreeze into it and it still overheats. I had the thermostat checked and the water pump and they checked out okay also. Please helpppppppp. Thank you.