Lindsay Lohan -- Face-to-Face with Probation Officer

2/2/2011 5:10 PM PST BY TMZ STAFF
Lindsay Lohan had some serious 'splainin to do at the L.A. County Probation Department in Santa Monica today -- during a sit-down with her P.O.

As we first reported, the LAPD contacted the probation office last night to inform them that they are  investigating LiLo for grand theft jewelry.

As you already know -- LiLo is on wafer-thin ice ... and the alleged theft of the $2,500 necklace could be a felony and grounds to violate her probation ... which probably means slammer time. 

UPDATE:  Sources tell us ... Lindsay was drug tested during her meeting with the probation officer but the subject of the alleged grand theft never came up.