Why would I only get chest pain when sitting a certain way?

Difficult to answer. If you are getting chest pain sitting a certain way it is possibly musculoskeletal in origin and related to posture. You may to go to your doctor to sort this out. Doesn't appear to be anything serious.
Bone pains? Posture related pains are usually due to abnormal joints or skeletal system. I would pay attebtion to the curvature of the spine. Seeing a physiotherapist may be of help.