Hi thanks for your query and welcome to lybrate. I am Dr. Akshay from fortis hospital, new delhi.
You are very young and if you have not sustained any significant shoulder injury, then this pain can be due to weakness in your muscles, or some bit of soft tissue strain or inflammation.
If there are no s/s of inflammation like swelling, skin redness or local warmth etc, then you can start with gentle anti inflammatory medications, ice packs for local application, analgesic spray after fomentation and see how it goes in the next 2-3 weeks, and if it does not improve, then we will have to get some investigation done for evaluation. Do not hesitate to contact me if you need any further assistance. You can also discuss your case and treatment plans with me in a greater detail in a private consultation.