Thank you for writing.
Is your pain localised to your shoulder? Or is it spreading down from your neck to your arm?
if the pain is localised to the shoulder, you could be having an arthritis of the shoulder joint, a ligament tear or just a sprain of the shoulder. In all these cases, avoid overhead activities for a few days and avoid lying on the affected side. If the pain does not improve please consult an orthopaedician with speciality in joints.
If the pain is spreading down your neck into your arm, then you could be suffering from a neck problem with referred pain in your arm. In this case, take due care on the position of your neck. Avoid high pillows and prolonged travel. Use a neck or head rest at all times. You might also need strengthening exercises for your neck if your initial pain improves. If it doesn't improve you need consultation with an orthopaedician.
Hope that helps.