19-year-old Leah is telling friends and family ... she had just left the theater with her fiance Sunday night when an older woman began following her and calling her names.

We're told Leah insists she tried to tell the woman to leave her alone ... when 2 other girls appeared out of nowhere and the three chicks opened a can of whoopass all over Leah's face.

Sources say Leah's fiance, and a security guard, managed to pull the three women off the MTV star ... and got out of the area.

TMZ obtained a photo of Leah's face following the brawl ... which shows a swollen cheek and redness on her neck and chest.

We're told Leah did not recognize any of her attackers ... but Leah's mother has since gone to Twitter in an effort to try and identify the assailants.