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.
Just to offer a counter anecdote, a friend of mine had the opposite experience. She'd struggled all through school, until her dyslexia was diagnosed, and she could be taught compensation strategies. After that, she was much more successful.
It's v. unscientific I know but it's an interesting observation all the same.