Brittny Gastineau and her boyfriend (Marquis Lewis) got into a fight which resulted in her getting a black eye ... and cops are now investigating the incident.

The reality star went out to dinner in Beverly Hills on Tuesday night and she had a shiner.

Sources say she and Lewis got into a fight at a hotel in West Hollywood last week.

His lawyer says she was the aggressor. She hired a lawyer as police are now investigating the fight.