Lowering a vehicle can change the geometry of the front suspension.
By lowering a car you can create a condition called Bump Steer.
http://en.wikipedia.org/wiki/Bump_steer
This was really noticeable in my 89 Mustang that was lowered 1.5"
You find that when going over bumps, you have to steer a little to over come the bump steer caused by incorrect geometry of the front suspension. It's a bit of a pain but you do get used to driving the car that way.
For the Mustangs, there is a bump steer kit that fixes the problem. It is essentially an extended tie rod end that corrects the alignment of the front suspension.
http://www.steeda.com/products/ford_...psteer_kit.php
Not sure what is available for your car.