Tough call. I tend to go though periods of insomnia followed by periods ( a week or so ) of fatigue and tiredness. Some days I can sleep for 16 hours if I wanted to. I blame the meds I am on, they are made to combat my insomnia but they tend to effect me for days at a time sometimes. Also I blame my activity level is not always so high. You might try exercise.
Or,
If you are in a stage where you are hurting a lot from anxiety, you might have anxiety breathing issues which is causing the fatigue. I had this for many months, it would hit me all day long as the anxiety persisted but was more noticeable when I would sit down, I would almost fall asleep in the chair and would have a hard time filling out the paperwork. It would cause a lot of dissociation also, causing me to space out. But as long as I kept moving and walking around I was 'okay'.