Nick Reiner Disciplined for Being 'Extremely Hostile' in L.A. Jail Shower: Report

Nick Reiner has reportedly gotten into trouble behind bars ... because the man accused of murdering his famous parents allegedly became "extremely hostile" with L.A. jail staff.

According to PEOPLE ... Nick got disciplined back in April ... after he got super pissed and started yelling when a deputy didn't come to get him fast enough when he was done with a shower to return to his cell. Nick was "screaming, using profane language and directing hostile remarks at both a sheriff and a sergeant" -- but it never got physical.

PEOPLE reports Nick was issued a disciplinary write-up ... and lost several inmate privileges for five days, including phone calls, visitation, and commissary access.

Nick reportedly made such a scene ... that he distracted guards while an emergency was happening elsewhere.