What REALLY Happened!!

X17 has uncovered the truth behind Brit's less than energetic

performance tonight.

Check it out -- we're all wondering why Britney looked like she was doing a

dry run rather than a full-on VMAs gig tonight and X17 has the answer!

A source tells X17 Brit found out about Sarah Silverman's crappy line about

Britney's babies just moments before she took the stage to perform "Gimme More."

Needless to say, Britney was PISSED! ... as any mom would be. Britney was so upset

she walked through her performance just to get it over with (despite encouragement

from her friends, including Sam who was apparently yelling at Sarah backstage to skip

out on it all together). She held up her end of the bargain and got a major slap in the

face from Sarah.

Chin up, Brit -- you'll rock our worlds when you're ready and Sarah's dating

Jimmy Kimmel -- how great can her life be...

No mistake, y'all -- these babies are adorable and Britney

Credit: X17 Online