'Dancing' Star -- I Got Rammed Before the After Party

Derek Hough was just trying to party with his fellow "Dancing with the Stars" castmates ... when he got slammed into from behind ... by some dude in a Porsche.

Hough was waiting to valet his car at STK last night ... when another driver banged into the back of D's Audi.

After the accident, the two exchanged information in front of the paparazzi ... and the giddy other driver couldn't have been happier ... sporting a big ol' smile on his face as the snappers took shot after shot.