Automatic Transmission Stuck in Third Gear

I recently had my timing belt replaced at a garage(1 week ago). They replaced the whole kit and cleaned the engine. I was driving for an hour at about 60mph and all of a sudden the transmission shifted down to 3rd gear. It was stuck in 3rd, so I pulled over and restarted the engine. It worked fine for two days and the exact same thing happened again after driving for about an hour. I pulled over and restarted the engine, but this time it was stuck in 1st. I manually tried to shift it, but it would only shift up to 2nd and back to first after numerous attempts. I put it back to auto and it worked fine again. When it was stuck in 3rd i could manually shift to 2nd and back to 3rd. It worked fine before it was worked on. Could this have anything to do with having it worked on recently?
1991 Hyundai/Santa Fe (50k)