Hello and welcome...
I wish we could help, but we would only be guessing at what could be wrong. The fact that the foot is swollen could mean infection. She may of been in a fight with another cat or animal. Get your kitty to the vet, then let us know how it went. And above all else, please don't let your vet talk you in to the long lasting CONVENIA injection. If kitty needs anti-biotics, it's best you go to the trouble (and I know it is) of giving them 10-days orally to her.
We will be waiting for you update. Good luck!
Just a tip...cats hide their pain/illness very well. It is in their nature. Although you kitty clearly has an injury, it's not unusual for her to continue to eat, drink and purr. Also, purring can be a cat's way of soothing themselves when in pain. If the eating stops for more than a day, this is an emergency.