Shoulder pain can be off various reasons, if it has started after long travel, probably a muscle spasm, can take muscle relaxants like tablet myoril 4 mg twice for 3 days, if the pain doesn't subside after 3 days, needs a clinical evaluation at hospital. Ice packs can help too.
Next Steps
take muscle relaxants with one analgesic like paracetamol for 3 days, if u have access to physiotherapy, try Interferencial therapy for 5 days