in the writing challenge she says "mes jambes"- this is a mistake?

Bonjour Diana !

No, this isn't a mistake: here it's a case of needing to specify which legs as the context is not that explicit.
For example, the verb here is not reflexive, as in "Je me brosse les dents." (I brush myself the teeth.), in which case we're given the owner of the teeth within the sentence.
Here we need to say "mes jambes" as opposed to someone else's.
Have a look at the related lesson:
Special cases when you use mon/ma/mes/etc with parts of the body (French Possessive Adjectives)

I hope that's helpful!
À bientôt !