I have a lump on my rib....Help!?!

I have for about the last 2weeks had a lump just under my right breast near the centre theres a lump on my ribcage (about an inch or so down from my breast) its hard and painfull but very small and hard to notice and i only noticed it when i was leaning on a table and it hurt so i felt it and there was a small lump about the size of a thumb nail, its continued to get more and more painful, i really hate going to the doctors so any help would be great. im 17 and slightly overweight. I havent taken anything strange i dont smoke or drink and live a pretty active life. Help!? p.s im female x