Alien impaled her
Only way to double confirm is to either ask Ridley and/or ask the original script writers and ask their opinion on how they wrote her death.
But like to think, the alien, while Lambert was paralyzed with fear. The Alien got it's tail, and either rammed it's tail through her ass into her mouth, or eh.. and/or, through her vagina through her mouth aka Vlad the Impaler style. Which alot of the games etc seem to suggest.
(Sorry for the crude terms, but these movies are for adults)