Recently I have been having great difficulty focusing on my work and staying undistracted. After working for 20 or so minutes, I get a headache which instantly takes my attention off my work and renders me incapable of continuing. I get 8-9 hours of sleep every night but seem to be very tired and unable to concentrate throughout the day. My diet is normal and I do yoga every other morning and walk around for at least 2 hours a day. I can't seem to figure out what these are the symptoms of.