If she banged it on something wouldn't there be a contact bruise or cut? If bit by something a puncture? It's been red swollen all day no bruising but we did play balloon volley ball 2 days ago. She's not complaining of pain and will only keep ice on it a short time.
https://www.mayoclinic.org/diseases-conditions/cellulitis/symptoms-causes/syc-20370762
Please take her to the doc NOW.
Or an internal infection.
It could be many other things. Does she have rheumatoid arthritis?
Be sure to report this to her family. I would have her doctor see her. imo.
There are so many others more qualified than me who will stop by to answer you.
I just did not want you to be left without any answer. Welcome Hattygirl.