Many years ago, perhaps even when I was as young as you, I had a period of pain that sounds suspiciously like what you describe. I eventually decided that it was due to a very slight change to my sleep habits, that allowed the shoulder on the side that was slept on to "slip out" a bit more than usual, and very slightly compromise the blood flow to the muscles involved. After a period of restricted flow, the muscles registered "pain" for several hours after, eventually become more or less constant.

In my case, I found that just "sleeping on the other side" simply produced the same pain in the other arm. The problem was that I was not experiencing the usual "sleep phase" changes that prompted occasional shifting of position, so that a relatively minor, and basically "normal," interference with the circulation persisted for unusually long uninterupted periods.

The condition may have been aggravated by a fairly minor weight change - with more of a different distribution of the lumps and bumps than a change in number or amount.

As soon as the divorce was final, When certain life/emotional "disturbances" were resolved my sleep patterns returned to normal, more usual minor tossing and turning resumed, and the pain went away and has not returned. (Although at my age there are always a few others.)

Normal muscle tone usually prevents significant displacement of joints and/or sustained compression of significant circulation paths. A sleep disturbance that causes you to remain "inert" in a fixed position for sustained periods can overcome the natural relieving movements, and in my case produced pain very similar to your description.

A deliberate change in sleeping position, and/or a shift in bedtime and/or wakeup time, or almost anything that might change the patterns of your sleeping might be of help. In my case, I suspect that a decrease in REM sleep frequency or duration probably occured, but that analysis can get extremely complex and is subject to vague and uncertain interpretations. Being completely and positively relaxed when going to bed might help - although there are specialists in the field who seem mostly to just make up theories in the absence of really understanding it all, and I don't have the theories to do any better.

John