Sounds like subluxation of the sternoclavicular. Perhaps, seeking orthopedic follow up is more appropriate? But your case doesn't sound like require surgical stabilization. Physiotherapy exercise probably cant help u much as the joint mainly uses ligaments to maintain joint position rather than muscles. Manipulation of the joint can be done by physiotherapists or chiropractor, but it will probably make the joint even more unstable and more prone to subluxation or dislocation, so that probably isn't appropriate either.
If I were you, I would get a consultation with orthopedic Dr first. Find out if it is anything serious. If there is nothing else that relate to the subluxation problem, then perhaps you dont need to do anything about it.