Christina Aguilera -- I Didn't Bruise Up My Baby!

Christina Aguilera is firing back at rumors she's somehow responsible for a giant bruise on her son's face -- telling friends little Max simply had himself a little fall at the park ... chasing squirrels.

The photo was taken yesterday afternoon at LAX -- with rumors circulating that Christina's "erratic behavior and alleged booze-filled outings" somehow contributed to Max's accident.

But sources tells us, that's total nonsense -- 3-year-old Max was out with his nanny last week, chasing around squirrels ... and the kid tripped over a rock and bruised his face.

According to our sources, Max was taken to the doctor immediately and everything was fine. No broken nose, no stitches. Just your average boyhood bruise.

As one source put it, "Christina is a great mom ... Max is her #1 priority."