My rattie has a bald patch with a scab behind her ear. I'm not sure what caused it as it must've happened when they were left with a friend whilst I was away. I also noticed her cage mate had really sharp nails when I came back, so I'm thinking she might have got a scratch when they were playing?
Anyway, fast forward a week since I noticed the scratch there's been a lump growing right where the scratch/scab is. Could this be just down to swelling and means it's healing or could it be something more serious? She doesn't seem to be bothered by it but I don't want to leave it be if it's something bad?

Thanks for you help!