Strange ribs

I'm a 17 year old girl, generally healthy as far as I'm aware, never had any major problems. The left side of my ribs sticks out what I would say as significantly more than the right side. There's no pain or discomfort just they seem to protrude further. When I lie on my side, I can also feel what seems like an extra bone in my back at the bottom of the left side of my ribs. It definitely feels like bone and there is no matching 'bone' on the right hand side. It's not painful at all and it feels similar in size to a rib. It just doesn't feel connected to anything and sticks out quite significantly. What could this be?
Thank-you

[silverfox67]

Extra ribs are not very common - but they do happen.  I have a friend who has an extra rib but her's is up by her clavicle.  Surgery is not required or recommended unless the extra rib causes trouble or pain.