Well, it definitely isn't normal.

If it's only caused by bumpy conditions, I would check:

1. Tire wear to quickly verify wheel alignment
2. Rear sway bar - is it still connected to the chassis?
3. Rear shocks - are they still working? Test this by opening up the trunk and pushing gently up and down on the trunk sill. If you push and the car still moves up and down, you may have a shock issue.
4. Wheel bearings or suspension bushings. Jack up your car and firmly grab one wheel at a time. Try to shake the wheel in a twisting motion. It should be rock solid. Try all four wheels.