That's not a sign of IBD. My snakes do the same thing if they are stretching themselves up really far. You might be talking about a periscope. Look up some vids on YouTube of a snake with IBD. They usually corkscrew like crazy and "stargaze". Is your snake still eating normally? Is the shaking the only thing that is worrying you? Is the snake acting normally besides the shaking? Sorry for all the questions, just want a little more insight