Lindsay Lohan -- Bodyguard Shopping After Trespassing Incident

Yesterday's door-knocking incident was a huge wake up call for Lindsay Lohan -- who's now in the market to ramp up security at her L.A. home ... with surveillance cameras ... and even a new bodyguard.

Sources close to Lindsay tell TMZ, the actress loves her Venice Beach house ... but with random scary people walking right up to her front door, she's convinced it's time to invest in some extra security upgrades for the property.

On top of adding additional security cameras -- we're told Lindsay wants to hire a full-time bodyguard to stay with her for a period of time ... until she feels safe again.

As for the man arrested for trespassing outside her house on Sunday -- Lonnie Short -- we're told he shares a bizarre connection with Lindsay's alleged stalker David Cocordan.

According to sources, when Short knocked on Lindsay's door yesterday, he announced he was friends with David and needed to explain some things.