Dawn,
My thoughts shared from my neuroligist were these; when there is compression, the blood flows into the arms, but the compression lacks the ability to make it flow out well. SO it stays there until it is able, if able to come out.....

Other thought too, some have lumph drainage problems and the fluid builds up.

But, between, compressions and tissue damgae that holds fluids, with increased activity and blood flow it could build up. Since my TOS, I had to have my wedding band set resized twice, two hole sizes bigger!

maybe there are ideas to help drain out the sweeling, I do take a mild diauritic, I avoid overhead work or using my arms the next day, like no laundry or ironing, hanging clothes, etc.
Avoid sleeping with your arms over head. If you have to think of a contraption to keep them from going up, do it.
Nite,
Di