Lindsay Lohan -- In Fear of Possible Stalker

Sources familiar with the situation tell TMZ Lindsay has been receiving harassing phone calls and text messages from unknown numbers -- and the unidentified person behind them has been making disparaging statements about her recovery ... and her family.

Even scarier, the unknown person is saying he/she knows where Lindsay is staying and is watching her.

We're told Betty Ford officials are so concerned, they're moving their most famous patient to a more secure, private facility.

We're also told BF has limited Lindsay's outings out of concern for her safety.