Hello Sir,
If lying on the shoulder is painful, when lying on it the shoulder structures may be irritating it.
Yes, knowing if you are a diabetic would predispose you to frozen shoulder issues.
I have described a few exercises for shoulder pain. Please check
https://www.youtube.com/channel/UCl1cWua2RplXZcbLO6hHdFw
Does your neck hurt too? It could a source too. If yes, in the above mentioned channel there are neck exercises too.
Ice is the option usually as if the symptoms are from the joint of your collar bone and scapula, it responds better to cold. However if the upper trapezius area is affected and since it has been a week use heat on the muscle.
Next Steps
Try the exercises mention in the link above ( these are for generalised shoulder restriction due to pain).
but an assessment is required to diagnose the root cause of symptoms
Health Tips
If there is burning irritation with heat try cold.