The pain appears to be muscular pull starting from mid back region radiating towards left side and shoulder. Was there any injury or jerk?? Do you have any history of any cardiac problem in family.. very less likely to be cardiac origin pain but possibility to be ruled out..
Next Steps
I will suggest you to start hot fomentation, take one analgesic with muscle relaxant combination, apply local analgesic ointment.
Health Tips
Avoid heavy work, avoid lifting any object, avoid forward bending.