Bam Margera -- Naked Chick Broke INTO MY HOUSE, Started Masturbating

According to law enforcement sources, Bam told police the 24-year-old woman woke him up in his Pennsylvania home by kissing him -- completely in the buff -- and Bam freaked ... throwing the girl off him and dialing 911.

And here's where it gets really insane -- according to law enforcement sources, while Bam was on the phone with cops, he claims the girl dropped to the floor and started to masturbate.

After Bam hung up, we're told he kicked the girl out of his house -- still completely naked.

West Chester police tell TMZ, they arrested her nakedness in the rural area surrounding Bam's mansion.