I just found this old thread and am glad to see that I’m not the only one who gets this strange internal shaking feeling. It’s usually a sign of me being more tired than I think I am and is a kind of warning to stop and rest. If I ignore it and carry on (unfortunately a common trait of mine!) I end up so tired that when I go to bed I end up actually shaking a lot all over and the more I try to stop the shaking, the worse it gets. Try explaining that to a nurse who wants you to keep still while she does an ecg! Trouble is I am getting the internal shaking a lot more often these days 
I just don’t want to admit I can’t just keep doing lots and not paying for it later (or rather sooner, these days!)