Thanks for choosing HCM.
You may have to stop over exerting the left arm.
the neck or the arm.
Hot water / Hot patch fomentation to neck will be beneficial.
Try using cervical collar
for 3 hours at a time 2-3 times a day.
Reduce the height of pillow.
Get some Physiotherapy
done from a qualified Physoiotherapist.
If no relief see a Spine Surgeon