Your anxiety is getting the better of your thinking process because it is making you over-analyze your body looking for reasons why you have weakness in your arm. You are naturally thinking this weakness has deep meaning because you noticed it at the same time as the anxiety, but there is no relationship between anxiety and muscle power.
Anxiety gets people so worked up that they imagine things are happening to their body or else they think something old has just happened now because they didn't bother thinking about it before. So my guess is that there isn't any muscle weakness anyway and you just imagined it, but if you are sure weakness exists then you should see a professional to find out. The fact it comes and goes likely means it is just in your head.
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