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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 ...