2000 sunfife rear brakes locking up when brakes applied

[pontiac35hp]

I have a 2000 pontiac sunfire i replaced rear drum brakes 6 months ago been working fine.yesterday i replaced front pads and rotors. heres my problem i backed out of garage and started forward and now when i apply brakes the car squats down in rear and rear wheels lock up as if emergency brake is on.rear tires are completely locked up until i back up.backing up seems to release rear brakes.going forward and applying brakes makes rear brakes lock up again.please help i worked on car all day yesterday (sat)and i need to get fix today i need car on road mon morning. please help asap thanks