Celebrity Rehab's Shifty Shellshock Busted For Rock Cocaine

Sources tell TMZ ... the former Crazy Town singer and his GF were arguing inside of the Ross on Sunset Blvd. in L.A. last week ... when things allegedly turned physical.

We're told a Ross employee believes he saw Shifty grab the woman by the arm and rough her up ... so the employee ran outside and flagged down police.

Sources tell us ... cops searched Shellshock and found rock cocaine ... and arrested the guy for possession of a controlled substance.