This may be due to lower neck muscle strain with radiculopathy towards shoulder region.. Sometimes, there is a possibility of cervical spondylosis which may cause similar pain..
Next Steps
You should take rest, start with hot fomentation, take analgesic with muscle relaxant tablet, apply analgesic ointment locally. If pain doesnt settle down, plan for x ray of cervical spine.
Health Tips
Avoid heavy work, avoid jerky movements of neck..