I observed something similar with dyslexia. A friend of mine is dyslexic. However he was only told that he was dyslexic a few months ago. Before he was told, he could read fine, and was quite good maths. Almost immediately after he was diagnosed, he started stumbling over words, and skipping sentences; he also dropped down to the 2nd last maths class from the 2 highest.

It's v. unscientific I know but it's an interesting observation all the same.