Transmission started shifting hard
No noticeable previous symptoms. Everything has been fine as far as I know. No warning lights either. My HHR had been sitting for 3 days in 100 degree heat. I go out to do some errands, start the car and wait a couple minutes. I put it into drive and the whole car jerked a little. I start to drive and it jerks every time it shifts. I make a stop at a store. Come back to my car to go home, and now it's shifting fine again. What could be the deal here?