When you inhale, you're stretching intercostal and pectoralis muscles of the chest wall which can pop and are worse with deep breaths.
The more you think about it, the worse it gets. Relaxation, heat, and analgesics help: Ibuprofen (like motrin) or naprosyn (naproxen) (like aleve) are good ones. Viral infections of the lining of the lungs or heart can cause pleurisy but usually with fever +/- flu like sx.
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