Could be a lot of things…..subacromial impingement, bursitis, rotator cuff tear, calcium deposits in tendons…I would recommend seeing your GP who may refer you to a specialist. Sometimes all is needed is some physiotherapy, or maybe a steroid injection into the shoulder. He may also send you for an ultrasound scan/x ray . You could try some anti inflammatory meds..eg Brufen/Paracetemol. What age are you? I am in no way trying to diagnose your shoulder problem but giving you some pointers. My hubby is an upper limb orthopaedic surgeon so he sees a lot of shoulders. Hope you get it sorted soon.