Suffering from episodes of lethargy, lack of alertness, no energy. Blood test done, normal. Symptoms occurring post menopause. Treatment?

i go through periods where i become very lethargic and not fully alert. i have no headache or any pain, just a feeling like I am about to fall asleep and no energy what so ever. i have recently had blood drawn not for this problem, and all of my results look good. it was a draw for colestrol. this condition will pass after a few days and I will feel good and energetic again. I am 68 years old, very active, have lots of hobbies and family activities. i have been going through these spells since I went into menopause . what is the cause of this so tired felling. I sleep pretty good and long hours at night.