I've noticed a big time day-to-day correlation between my chess performance and the amount of sleep I had. Also, coming back refreshed from a 1 or 2-week vacation (with plenty of sleep) leads to an immediate boost as well.
Sounds like "The yips". I know what you mean. It's as if overusing part of the brain makes the connections less powerful than they'd normally be. Possibly already some credible science behind it.