Transmission hard shifting first to second gear

I have a hard shift from 1st to 2nd during my first 2-3 miles of driving when the engine is cold in my Lexus IS 350 (Sorry I didn't see a 350 option in the menu so I put 250). If I get to about 2,700 RPMs or higher it feels like my car got a slight nudge from a car behind me. The transmission quickly shifts into 2nd but abruptly, no clunk or any strange noises. It shifts into drive/park/reverse perfectly fine and all the gears shifts buttery smooth when it warms up. I had my transmission fluid changed, which was near black colored and the problem still persist. No slipping, no check engine light, and they shift at the proper time. Any idea? Thanks!