Lane Garrison -- Cheating May Have Triggered Domestic Violence

Lane Garrison's girlfriend told him she had been cheating on him during their 8-month relationship, and that may have triggered what appears to be a brutal confrontation later that day.

Law enforcement sources tell us ... according to the police report, the night before the incident, Lane got drunk at the Playboy Mansion, and got thrown out. Lane's girlfriend, Ashley Mattingly, was also at the party and apparently the incident was the final straw for her.

The next day -- last Saturday -- Mattingly texted Garrison that she was breaking up with him and confessed she had been "seeing someone else" while dating him.

According to the police report, later that day, Garrison went to Mattingly's Bev Hills apartment, waited for her and when she arrived he said, "Baby I love you. Come here." They began to argue and according to what Mattingly told cops, Lane grabbed her by her hair from behind, threw her to the ground and began punching and kicking her in the face and head.

The surveillance video (below) -- obtained by TMZ -- only shows one physical incident -- where Lane appears to slap Ashley. But Ashley's account is backed up by 3 witnesses who spoke with cops. According to the police report, one witness said he saw Lane punch Ashley in the face and head. The second witness told cops a similar story.

A third witness -- Stephanie -- told cops she saw Lane bang Mattingly's head on the mirror and then punch her 4 times with closed fists. Stephanie says Lane was screaming, "F**k you bitch. You cheated on me."

Ashley says the day after the incident Lane called her and said he was going to kill himself.

Lane also spoke with cops and claimed he never struck Ashely during their relationship.