All of my three children own rabbits. And this morning, while I was stroking one of them I found quite a lump on its back. I did noticed they had been fighting and they do share a pen. Could this have anything to do with the lump? I can't see an obvious wound or anything like that...
Yes, they can dig their nails into each other, which can leave almost unnoticeable marks that can infect and in time cause really nasty lumps. I can't tell from just your description if this is indeed what has happened, but if it has, you need to consult a vet, so I suggest you get on the phone and make an appointment ASAP!