salsalicious

I have an RX300 year 2000 and for some reason whenever im driving it sounds like I have a flat tire and when I speed up like on the highways my car starts shaking a lot!! I took it to some place to check it out and they said I need to replace my back passenger side rotor??? Im totally lost, how do I know if this is true. I think I may have hit a curb a couple times, would it mess up the rotor? Please help me :cries: Thanks!!