What Causes Painful Spot On Rib?

I have a spot to pop out on my rib under left breast an it was dime size tester day when I noticed it it has a place with real small scab so I slept with heating pad on it last night an when I got up this morning it was 5times bigger an is painful to touch it is hard on the top an soft underneath could this be a boil an the heat is just raising it to bring it to a head ?