Based on the information provided it seem like you are having pain in left shoulder blade and you are sure that you do not have done any strenuous exercise and wanting to know the cause of the issue.
Their are two possible cause for pain in left shoulder blade- 1) issues with lungs upper border and 2) Pancreatitis
and the most common cause being strenuous exercise is ruled out from your history.
1) Contact near by physician and talk about the issues you are facing.
2) I recommend X-rays chest to rule out the lungs issues, and
3) Needs abdominal Ultrasound
to check for pancreas
issues if any.
Hope it helps