Ok, If I am thinking about how your describing how your arm is anatomically speaking. Then we are talking about your wrist extensors. If it is in fact your extensors we are talking about here, there could be several causes.
One is that your scalenes in your neck are very tight from being immobilized from the surgery and the arm sling. This can cause compression on the brachial plexus because this plexus runs between the middle and anterior scalene muscles. ( I think this is unlikely since you would have other neuropathies)
The Second and the most likely in my opinion is that you have some sort of compression or damage to the posterior cord of the brachial plexus. This eventually branches off to form the radial nerve. Which controls the wrist extensors and triceps.
Do you have and tricep dysfunction by chance.?
Keep in mind this is if I have your arm position down correctly from your description.
here are some links to look at that may help with your descriptions and some of my explanations.
http://depts.washington.edu/anesth/r...usanatomy.html
http://www.nlm.nih.gov/medlineplus/e...cle/000790.htm
Disclaimer: this is for informational purposes only and in no way should this be used instead of a doctors advice, but you already know that.....LOL