thanks for writing to HCM,
your symptoms can be due to few factors - severe trapezius spasm
with trigger points, cervical disc related problem, postural pain leading to stiffness
of neck and upper back. you need to undergo a detailed physical examination to find out the cause, & if required some lab investigations also like MRI cervical spine
I would suggest you to see a physiotherapist because in all the above mentioned cases he/she can help you. since heat helps you definitely shows there is muscle spasm so a therapist can give you some heat therapy
, intermittent cervical traction and neural mobilization of left shoulder and arm. and gradually some stretching and strengthening exercises and postural correction advice.
some do's and dont's for you
do not use pillow while sleeping, do use a towel wrap (very thin) under your head while sleeping.
do not lift any heavy weight.
continue hot fomentation
do not work on computers continuously for long hours, take break of 2 mins every half an hour.
i hope this helps
wish you good health