being somewhat "off [n.b. not 'out of'] her form

Ah yes Annati, yet another variant..

"Out of form" would definitely be a sporting (especially horse-racing) reference this side of the pond.

"Off form" is quite close to "off colour" (i.e. ill, implying a temporary aberration), but is mostly a reference to social behaviour. Someone who's off form is "not themself" at the moment.
You could talk about, say, a horse being "off form", and would be understood, but there's still an implication of unusual (and disappointing) behaviour over and above the horse running its races like a donkey.

"Bad form" would be extremely - possibly irretrievably - off form. This would never be used about anything other than someone's behaviour.

Surprisingly rich area, this.