Rihanna has a little birthday fun!!

She's so sexy-- and i'm not talking about Rihanna

Rihanna celebrated her birthday on Saturday night with boyfriend Matt Kemp, a few close friends ... and a little person who gives lap dances.

It all went down in Arizona ... during a weekend in which RiRi went barhopping in Scottsdale.

What more could a girl want?
UPDATE: The tiny dancer is "Bridget the Midget," a porn star who also appeared on "Cathouse."