Britney Spears Dances for the Camera After Saying Her Name's Now River Red

Britney Spears -- or River Red, as she'd like to be called -- swapped outfits 3 times Sunday night, dancing to her heart's content ... just days after making the apparent name change.

The singer showed off her signature moves Sunday evening ... grooving to 3 separate tracks, complete with costume changes, too.

In her caption, Britney says she can't act "too kooky or silly like kids" because people will call her "CRAY CRAY" -- adding she's sharing her dancing because "every person I call TAKES 9 RINGS TO ANSWER" ... and, "I bet after I post this my security answers after 2 RINGS."

The freestyle session came after a now-deleted post from last week, showing off a picture of Mars and writing, "I changed my name to River Red" ... and saying "don't underestimate the power of purity."

She also changed her name on IG to match the announcement.

Fans are scratching their heads at the supposed name change -- we are too -- but it might simply be her take on Mars, known as the red planet.

There's also a little known 1998 film called "River Red" -- a disturbing family drama -- but that seems much less likely.

As we reported, eyewitnesses told us Britney's hubby Sam Asghari stormed out of a dinner in Woodland Hills this month when she was acting "manic" at their table.