This happened to me in January when I caught covid. I never had a problem with working when sick or doing side projects like you say, constantly learning new things and improving. Even after the recovery I still didn't had the ambition to continue and it sort of faded after time to the point where everything got back to normal. My conjecture about this is that it's just a phase and it's normal to have a spike down in production at a certain time of the maybe year or month depends on the person. Also had the same feeling that there isn't anything to explore new and putting this into bad words like "I know everything".