transmission acting werid..

[94502]

I was driving home tonight going about 30-35 mph accelerating steadily and suddenly my car felt like my car has been hit from behind lightly.. It happened really fast, I look behind me but I couldn't have been hit, so I think its my transmission downshifting for no reason.. I was just driving normaly not gasing it or anything, just with my foot on the gas pedal to maintain the speed. Has this happen to anyone. This is my first time experiencing this, but I have experience the lurch from a stop then accelerating. I recently brought the car in for the lurching problem and they fixed it, i think they did an update or something. After that the lurching problem was gone, but every now and then I get it again the lurching was not as abrupt as before..