This could be a number of conditions, but I would be concerned that it might be a form of skin cancer. If so, it could be a benign one like a histiocytoma, or a more aggressive disease like a mast cell tumour. The best thing to do would be U.K. get it checked by your vet.
Take a look at this blog for some more thoughts... https://vethelpdirect.com/vetblog/2019/11/04/my-pet-has-a-lump-what-should-i-do/You must be logged in to create new threads, or access some of the forums