Sharp pain traveling from upper left back right below shoulder blade to under the left breast leaving a lingering pain as it travels. what could it ?

Neuralgia: Pain radiating from under the shoulder blade around to under the breast may be due to compression(pinch) of a thoracic nerve as it travels along the under side of the rib. Sleeping in an awkward position is one cause. The pain can be severe. It's usually a temporary problem but can last for several weeks. If OTC NSAIDS don't help gabapentin can be very effective.