1999 Mazda 626 4cyl Overheating

A couple of days ago my car overheated on the freeway. The heater stopped working so I noticed that the temp gauge was hot. I stopped at the gas station, let it cool, and filled it with coolant because it was very low. I let it idle for 20 minutes and it was fine so I began driving again on the freeway. It started to overheat so I got off. As soon as my speed dropped below 60mph, the temp gauge dropped so I drove home on the back roads.

The next day I tried driving it again and as soon as I got above 60mph, the temp was hot. No smoke and I had the heater on blast. The way home was in stop and go traffic and no overheating. My mechanic wants me to bring it in on Monday, but I'm trying to find out if anyone else has had this problem. Hoping it's a cheap fix. Any suggestions!