DkSlateGXP

So I recently purchased a 2008 Grand Prix GXP with 70k and when driving is home I noticed the suspension had what is to me an unusual bounce when I stopped at lights. When I brake and the vehicle comes to a stop the car bounces like someone was sitting on the back end and just got off. As I am new to this vehicle I am not sure if this is normal as is is not very pronounced? Any help is appreciated. Also I apologize in advance if this topic is covered somewhere else but I couldn't find it.