Can be foreign body.
This might be many things.
It can be just irritation, or it can be from an injury to the chest/ribs, etc. It might be pneumonia
but there would be fever
, etc. Another possibility is that it could be a foreign body
that was accidently swallowed and is stuck somewhere in the lungs/windpipe. If this has been going on for a while, you should see your doctor for an xray to be sure nothing is there.