'Celebrity Rehab' Star Shifty Shellshock -- Arrested for Rock Cocaine

Former "Celebrity Rehab" star Shifty Shellshock was arrested for possession of rock cocaine last night ... after getting into an argument with his GF at a Ross Dress for Less ... TMZ has learned.

Sources tell TMZ ... the former Crazy Town singer and his GF were arguing inside of the Ross on Sunset Blvd. in L.A. last night ... 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.

We're told Shifty is still in police custody.