I'm soooo scared.....SAT?!?!?

Variations do happen often frequently, as the test isn't perfect. (Or maybe it was just a bad test day for you.) I know several friends who've had major score jumps due to test variation. You seem like a very strong writer and based on your past performance, I wouldn't worry about it. Even if your skills did drop, the writing section is very easy to improve on. (it's mostly a test of your knowledge of grammar / concision)