This from someone on another forum (who I have no issue with believing)
Just to hopefully re-assure everyone, it is something she’s suffered from a lot previously in terms of panic attacks and not being able to return… so likely to be that. I know a few people in her team will update properly if the news doesn’t get there first!
So hopefully it is something she can overcome. You'd hate for such a talented player to be affected long-term by something seemingly out of her control. The fact she's suffered from it before and still came out to play, very well at that, on the grandest stage and court of them all is still testament to her character.