Why does my SAT score increase by 400+ points w/ out studying?

I've always struggled w/ standardized tests, gotten sporadic scores that seem statistically impossible. (got sat 1600 then 2100 without studying) I think that i might have this type of dyslexia that turns "on and off" depending on my stress level. It is also more severe when numbers and letters are in the same sentence. It's kind of embarring because i'm a good student.