Christina Aguilera TWO-FACED

9/19/2013 9:30 AM PDT

Christina Aguilera -- TWO-FACED

Christina Aguilera flaunted her hot new bod last night  ... but it wasn't her body people are talking about ... it's her face --  because it looks ... well, different.

32-year-old Xtina has shed what appears to be scores of lbs ... but the big change is in the punim.

She didn't explain how she did it last night on "The Tonight Show with Jay Leno." 

So we gotta ask...