Ja Rule interrupted Ashanti's set Saturday night and got the crowd to wish and sing her "Happy Birthday" ... this is, her birthday was 2 weeks ago!
Ja and Ashanti were both featured performers at Nightmare on Q street, at a concert thrown by radio station Q100 at the Orleans Arena in Vegas.
Ashanti was in the middle of her set when Ja Rule jumped on stage to wish her happy birthday and got the crowd all amped up. He got them to sing "Happy Birthday," and people in the audience did the OG version ... to Ashanti's delight.
Others performing at the concert included Bone Thugs-N-Harmony, Chingy, MIMS and Baby Bash.