If it's not hot, she's not lame, and it isn't painful to the touch I wouldn't be really worried about it. I'd call out the vet though, if it doesn't go down within another week, as anything with joints can get a little dicey and if it's bad the longer you wait the worse it could get. As a little note my mare just had a large swelling on her stifle that wasn't hot or painful. The vet determined that it was a hematoma and drained it. It would have gone down eventually, but this was faster. Good luck and a quick recovery to your mare!