It sounds although you may have a tendonitis
. This is inflammation in the tendons around the shoulder. It is very common as the shoulder joint is like a ball on a saucer and all of its stability is offered only by the muscles and tendons around it. This may improve spontaneously with rest and anti-inflammatory medication but if it does not then an Xray and possibly an injection may be of benefit to you. If this does not help then further investigation with an ultrasound
or an MRI
scan may help make a diagnosis and lead to recommendations regarding further treatment.