There can be a number of reasons for such pain but prima fascia it seems to be either muscular spasm or trapezitis.
For detailed diagnosis and proper treatment plz visit my clinic, so that I can help you in a better way.
Next Steps
To relieve ur self plz start with hot fermentation(moist) and any pain relieving spray .
Health Tips
1- avoid hanging hand, keep it supported in pocket or with other hand
2- Avoid prolonged sitting n standing
3- Avoid carrying wt in hand.