Lump on male rats rib cage?

Question
Hi

I recently found a small lump on my male rats rib cage which is almost certainly under the skin (i.e deeper than an abcess) as one of my other rats had an abcess on his chin a few months ago and it doesn't seem anything like it.

Will it most probably be a tumour or is there another possible explaination? I'm going to try and get him an appointment with the vets tomorrow, as I am really concerned.

Thank you

Answer
Can you see it protrude or just feel it during palpation of the area?

Is it toward his underarm?  It is probably a mammary tumor if it is in that area and it can be removed if it isnt involved with the rib cage.

If it came up suddenly, it can be an abscess but it is internal, which is not unheard of.

Let me know how the vet visit goes.