What Could Cause Intermittent Symptoms Of Tiredness, Body Soreness, And Trouble Focusing With Prior Diagnosis Of Lyme Disease?

76 year old active male in fair condition. Lately have experienced what I am calling having a good day and then a poor day. For me yesterday was not good. More tired, body soreness, trouble focusing dozing while seated in afternoon and evening. The VA has checked for auto immune disorder and that is negative. I had Lyme disease in June and was treated. What situation would cause me a good day and bad day and back to good? Thanks