The key to comfort at any sitting task is to make sure you're at the correct height. I get the same kind of issue when I work on a laptop at a table rather than a desk. Make sure your forearms are at a 90 degree angle to your upper arms when you're sewing. Hopefully you have a chair with adjustable height so adjust it appropriately. Also make sure you are sitting directly in front of your machine. Occasionally I've seen sewing cabinets where the machine is off to the side a bit - they always looked like pain to me!
Pam