Help, australorp limping

Even a vet may not be certain. But if she is an adult, she is a pretty heavy bird, and they can injure themselves umping off things rather easily. She could also have some sort of arthritis or other joint problem, and of course there is always tumor. But the most likely thing is some sort of temporary sprain or strain type injury which will hopefully respond to rest. I would try keeping her quiet for a few days, or even a few weeks. This may mean separating her and putting her in a somewhat darkened place. Don't let her jump down from a roost. If your roosts are high, say over 2', she may need a lower place to sleep, or even a spot at ground level, at least for now. Certainly a good number of these do resolve themselves, and we never know what was wrong.

I would also check her a couple more times over the next few days for swelliing and warmth, which means inflammation, possibly infection. Sometimes they symptom is not so apparent at first.

Here's hoping you never find anything, and it just goes away!