Lump on ducks side, please help

[mrplattypus]

Have a female who has developed what appears to be a large lump on the left side of her breast. Its hard but doesn't appear to be giving her any pain. I don't think it could be a tumour or anything because surely I would have seen it growing? I thought possibly an abscess. Please help, i was thinking of just leaving it to see what happens as she is in no pain but i am really curious to know
what this could be. Thanks in advance guys!