Overheating?

I had a new radiator put in two years ago and have had no cooling system problems. Two weeks ago, I noticed my temperature gauge elevate. The next day, my overflow jug took 3/4 gallon of antifreeze. My mechanic put dye in my overflow but did not see anything showing a leak. He also did a pressure test on the radiator, and everything seemed fine. He put double the amount of dye in the overflow and told me to return after thanksgiving. Today, a week later, my temperature gauge went towards hot again. After the car cooled, I added about a third of a gallon. I haven't driven it and plan to take it back to my mechanic to take another look. He said he hopes it isn't the start of a head gasket leak and the coolant may possibly be going out my exhaust. Do you have any ideas as to what my problem could be? Thank you.