What Causes Headache, Tiredness And Difficulty In Thinking Clearly?

Hello! I have symptoms of not thinking clearly and tired like I am not all there, for the past two weeks now? I am having trouble at work completing a thought, in a daze feeling. I can sleep 15 hours and still the same. With that I also have a constant headache. I take Tylenol or Advil and it helps my head but comes back as strong after a few hours. I have had a blood test and urine samples taken but nothing is showing up. Can you help me?