Thanks for posting your query.
According to your description , pain below shoulder seems to be inflammatory in nature.
I advice you to give you rest to your shoulder and arm, take anti-inflammatory drug like motrin
1 tablet with food as and when required (upto 4 tablets daily) to reduce pain and inflammation and consult your doctor for thorough examination of shoulder and arm and rule out any infective cause.
Wishing you good health ..
Dr Saurabh Gupta