She may have a big splinter or thorn in it. Or she could have pulled something in the leg if she got it caught somehow, possibly even just a torn toenail. Guineas are notorious for hurting themselves and can and do just all of a sudden coming up with unexplained, unobvious mystery injuries (as I am sure you know, they are not the brightest of poultry). Keep an eye on her and watch for any swelling, injuries or forming abscesses in the next few days, but most likely she'll baby her leg until she's healed up and back to normal. If you do eventually find an injury/wound on her, wash and medicate it then keep an eye on it to make sure the others do not cannibalize her wound. I swear by Blu-Kote as a great antiseptic and it always works great as far as hiding wounds and scabs.