EXCLUSIVE

Christina Aguilera is not in the Christmas spirit ... she spent her birthday in "The Happiest Place On Earth" ... in a heated argument with Mickey Mouse ... TMZ has learned.

Our theme park source tells us while ringing in her 34th b-day at Disney California Adventure, Christina and her crew tried to get a pic with Mickey, but he was going on a break ... so Christina was told she had to wait. Cue meltdown!

Aguilera lost it, and called Mickey an "a**hole." We're told she even dropped the dreaded, "Do you know who I am?"

Xtina's crew started hurling threats, we're told ... and Mickey had to be ushered to a safe zone -- out of public view. Security was called, but by the time they arrived ... the former Mouseketeer had already split.