If this has been happening most of your life, you may have SAD, Social Anxiety Disorder. It affects people when they must address a crowd or walk into a party where they don't know many/most of the people.........any situation where you feel everyone is staring at you. This would obviously apply when you are standing on a stage and everyone really IS staring at you! And in your case, not only staring but judging! Makes me shake just thinking about it!
Or you could simply have stage fright. Very few us feel comfortable being
"center stage," and we react in many ways. We break out into a sweat, we turn beet red, we begin to shake, our mouths turn dry as dust, we have coughing fits, we want to run away, we feel like we might faint..........the list is quite long. Many famous actors have horrible stage fright................
But your question was "could this be nerves or anxiety?" I would say that if shaking in situations that make you nervous is the worst thing that happens to you, then you're dealing with simple "nerves." In which case I would recommend you find some good relaxation techniques to use in stressful situations and before you audition. Many of these techniques can be done without anyone even knowing your doing them.
I doubt you have "anxiety" as we know it on this forum. Your symptoms would be much worse than just shaking and you would, literally, be unable to audition at all. In your case, I can see the words "nerves" and "anxiety" being interchanged. You could say that you feel "anxious" before an audition (or any stressful situation) and that would not mean you had an "anxiety disorder," it would simply mean you were incredibly "nervous."
If this shaking becomes a real problem and begins to interfere with your ability to enjoy your life or audition, then I would suggest a discussion with your doctor.
Meanwhile, take some deep breaths and sing your heart out! I wish you the very best.
Peace
Greenlydia