Thank you for clarifying,
The symptoms you describe are consistent with a problem with your rotator cuff.
The rotator cuff is made up of the tendons of the four muscles that move your arm.
It can become irritated/inflamed or injured for many reasons (repetitive movements or arthritis, for example). In your case, you may have arthritis that was present and not symptomatic until you were laying on the arm, causing irritation.
An orthopedic surgeon who specializes in shoulders can diagnose and treat your shoulder pain.
Initially, the treatment is typically anti-inflammatories and physical therapy exercises. Steroid injections may also be helpful.
Here is some information for you to read if you would like more description:http://www.medicinenet.com/rotator_cuff/article.htm#3howis
Feel free to ask any follow up questions you may have,
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