It could be a bite or they could have just stepped on something and it got swollen. Or it could be an infection. Watch for any telltale signs of infection, and in the meantime I would soak in epsom salt and keep it clean.
I agree with Araucana. I've had a couple chickens with this in the past. It has been infections both times. If a salt soak doesn't work you might try lancing it. If you aren't comfortable with it you might find someone who is. Hope it works anyway.
Thanks! Glad to hear that it doesn't sound like bumblefoot. I'll tell my next door neighbor to soak it, it's her baby. She was putting antibiotic on it earlier. Think she should continue that?