What Causes A Lump In The Chest Area On The First Rib?

Hi there,I was hoping you could help me. I m a 22 year old female in quite good shape (5 4.5 , about 128lbs) I noticed some months ago that on the left hand side of my chest, there is a noticeable lump underneath my clavicle on that side. The whole left side of my chest in this area is somewhat raised. The lump feels like it s my first rib as it s hard, and bone-like, it just seems to be protruding more than my other rib. It s also marginally higher up than the one on the right hand side and feels quite thick/larger than the one on the right also. It is not painful and does not cause any discomfort, but it s quite visible. I really noticed it the other day when I was trying on clothes in a changing room and saw it from an angle in the 360 degree mirror so I panicked. Have I got anything to be worried about? I m not sure if it always been like this - I lost weight shortly before I noticed it so I may have always been like this, but as I ve really been paying attention to it recently it s becoming more and more noticeable to me.I appreciate any help you can give me.Alison