Hi Earl,
I have RSI /TOS {repetitive strain injury/thoracic outlet syndrome} and when I get the pain under the shoulder blade - it's in the rhomboid muscles.

They can get easily overstretched - usually due to a forward shoulder posture. {computer use}

When you get that pain again maybe try some gentle stretches and posture awareness , bringing shoulders back to relieve the pulling on those muscles.

You can even lie on the floor and let gravity help so those shoulders lower back down into a better alignment. You can use an exercise ball , foam roller or even rolled up towel under the spine to help shoulder drop even more - it also opens up the chest muscles if they have shortened due to long term forward posture.

It feels good too!
and if it relieves the soreness after a few times then you know the source of that particular pain...

I just wanted to mention it in case it is something that helps for you or anyone else having similar pain.