Lindsay Lohan -- Allegedly Involved in Nightclub Altercation

Lindsay Lohan has been off formal probation for less than two weeks and she's already being accused of getting into an altercation with a woman in a nightclub earlier this week ... TMZ has learned.

According to our sources, a woman filed an incident report ALLEGING BATTERY with the West Hollywood Sheriff's Department last night. We're told the woman claims Lindsay got into it with her at a nightclub on Thursday night. She claims Lindsay did not like the fact she was talking to a male friend of LiLo's ... and began pushing and shoving her.

Law enforcement sources say they will investigate the woman's claims -- like they would any other incident like this -- to determine their validity.