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.
Please take her to the doc NOW.
https://www.mayoclinic.org/diseases-conditions/cellulitis/symptoms-causes/syc-20370762
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.