Can't focus studying at ketone diet! Help!

I've been in keto diet for almost 2 months now.  It's been really good for me in studying,  I can focus so much and my appetite for studying is really high!  Like I can study for 12 hours,  I'm currently preparing for my board exam.  Then suddenly,  I noticed this past week,  I can't focus studying,  I just keep reading my notes 5 times and still nothing comes in my mind,  I'm just staring at my book and notes and realized it's been an hour passed by.  I'm so bothered.  I feel like my mind is floating,  I'm still eating the same food,  please help if somebody do the same!