i am driving on the interstate when this happens.
why does my car shake when i use the brakes? on 2005 Pontiac Vibe
It needs to be properly diagnosed could be many things such as worn suspension, tire related issues but it is most likely due to the front brake rotors being "out of true". The rotors warp due to the heat of braking and will need to be measured to see if it is possible to machine your rotors or determine if the rotors themselves need to be replaced.