Christina Aguilera -- SURPRISE! I'm Pregnant Again

Christina Aguilera knows how to keep things interesting -- it hasn't even been a week since she announced her engagement to a production assistant ... and now she's pregnant.

Christina and soon-to-be baby daddy/husband Matt Rutler met on the set of the 2010 film "Burlesque," where he worked as a P.A. And things heated up from there.

Aguilera announced they were engaged on Valentine's Day.

Christina gave birth to her first child -- Max Bratman -- in 2008.

Pretty crazy timing ... Aguilera lost a ton of weight just last year.