What Causes Extreme Fatigue Along With Headache?

Yes, my symptoms are extreme fatigue and tiredness. I have slept 20 of the last 24 hours two days in a row. I have A slight headache, brain fog and I am really having a hard time concentrating. This set of symptoms has occurred twice in the last month and I have experienced it three times in the last 4 months. I have hypothyroidism and asthma. I have had my thyroid and hormone levels checked and they are both within the normal range with medication. When the symptoms let up I go back to my normal routine and seem to be just fine. I am concerned of the frequency that the symptoms are occurring now and don t understand why they are occurring. I am a fairly healthy 57-year-old woman, I go to the gym three times a week, when feeling good, I don t drink smoke have taken on sugar and gluten out of my diet for 3 months now. Any suggestions or ideas would be gladly appreciated