If you're using Slick2D to create the rectangle. Slick2D has a Shape class that can be used to describe such things (rectangles, circles, etc.).
I imagine you're using a shape like this for collision because it has methods like
intersect()(link) that allow you to detect such things.
Shape also has a nice method called
transform()(link). It can be used to apply a transform to your shape. Transforms are essentially matrix operations. Luckily you don't need to know too much about the matrix operation because it's easy to create transforms.
Transform has a static method called
createRotateTransform. You can use this to specify a rotation to create a transform to apply to the shape.
So the short of it would be, use something like this:
rotatedCarShape = carShape.transform(Transform.createRotateTransform(radiansToRotate)));
transform does not alter the original shape, so you don't have to worry about the rotations adding up, but you do need to remember to draw the correct transformed shape.