My brakes are grinding all of the sudden...why?

ok so about a week ago i had my highway tires replaced at sams club for all terrain tires on my truck. Before this my brakes were amazing....completely fine never once heard a squeak, squeal, and or grinding. now the day after they changed them i heard a grinding noise and not just a little grinding i mean alot. whenever i slow down and i get to like fifteen and under they grind a ton! and when im actually driving they rub as i drive u can hear it its like a knife sharpeningg stone getting rubbed on a knife in circles... i know it could be my pads being low but last time it was that it was a gradual increase in grinding....this leads me to believe they did something... please help!!!