If you say you have seen a doctor about this and there was no diagnosis made, it is possible that you just have a concave chest, which is usually due to a genetic predisposition, if anything it will probably make your pecs look bigger :). If you're having any difficulty in breathing or changes in breathing then you should see your doctor again to rule out any other possibilities.
It's possible, but it's not something you should be worrying about all the time. It may or may not happen, but there really isn't much you can do about it either way. You just have to let your body mature on it's own.