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Jessica Simpson is feeling the burn!

The singer, 29, complained that she overdid it after a Wednesday workout with personal trainer Mike Alexander.

See Jessica Simpson's most outrageous moments ever

"Why am I still hurting from our workout earlier?" she Twittered to him. "I'm getting old! See you tomorrow with a pouty face and sore buns."

Alexander tried to inspire her.

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He Twittered back that she's "a badass" whose "efforts know no limitations. And I can deal with looking at that pouty mug..."

This year, Simpson has come under heavy scrutiny over her weight. An Oct. 11 Fox NFL Sunday spoof apologized for joking that she would make a good "defensive tackle."

Find out what other stars -- like Jessica -- are tired of all the fat jokes!

Simpson Twittered back, "I will never understand why people attack for a laugh."