New pain in lungs

Not sure about sharp pain, but when I had some persistant coughing and lung discomfort my onc had me get a chest CT to check for pneumonitis, it was negative, nothing showed up in the scan. When was your last scan and when is the next one? I wouldn't worry about your onc immediately taking you off treatment but they'll want to investigate why you're having the pain, so maybe an xray or CT will be done. It might be something unrelated to treatment or melanoma completely, but it's best to get it checked out if it's been going on for a few weeks.

Curious as to whther you feel the pain is "in" the lung, or in the general location (and felt primarily in the rear / your back ).

If the latter, there are primary muscle groups in this area prone to spasm / strain and capable of causing prolonged discomfort which may mimic more serious conditions (dull persistent ache which can spike to very sharp pain upon movement). If muscular, you may find relief via warm heat (if this helps, then you may not need to worry).

However, 3 weeks feels like a long time for any muscle-related strain / spasm and a visit to your internist,, if not your onco may be a good idea.