2003 Mercury Sable shaking when breaking

I'm just wondering. We just recently replaced the rotors and the break pads in hopes of stopping the shaking in our car. When I break to come to a stop at a light the car shakes all over. I don't notice it when we are driving normally. (It could be there I just don't notice it) Do you have any idea what it could be? Alignment, balance or something else. Thanks Rachel Humphrey