Hi,
You don't say if the lump is on the breast itself,or inside your breast.
If it's inside your breast,it could be a cyst,since cysts can at times be quite painful.But of course you need to make sure of that, by having your doctor evaluate it
If, on the other hand the lump is on your breast,then it could be an infection, perhaps a furuncle (boil).Apply warm compresses and see if it drains on it's own.