Slight pain around shoulder blade when i exhale. should i visit the er? what could this be?

Uncertain: This could be something as simple as muscle but it also could be from the underlying chest. With the chest pain we can also worry about blood clots. You need to see your physician to clarify all this.

Likely musculature.: Likely a muscular injury . If you fell you might have a fracture.

Yes: Sudden onset pain with respiratory component could be pulmonary embolism. Unless specific incident, accident, or inciting event occurred, evaluation soon would be prudent. If recently ill with cough etc then chance is less likely for serious issue. Recent lower extremity pain such as in the calf could indicate a pulmonary embolism which can be life threatening.