Something that has developed so suddenly could very well be some type of infection; an abscess comes to mind and you wouldn't need to exhibit a fever or other such symptoms with this type of local infection. Pain is rarely associated with Breast Cancer but I would suggest that you see your Dr. for an exam and opinion as to what this lump represents. I can assure you that it most likely will NOT just go away on it's own and the sooner some type of treatment is started the sooner you will have some relief. Regards ..
could also be a cyst or boil