I would recommend getting tested for any sort of disease/infection that could produce these symptoms including/especially Lymes.
Backstory
I was a vegetarian through high school and didn't balance my diet correctly ending up being anemic for the first few years of my undergrad/end of high school. I than began eating meat again and balancing my diet more appropriately with iron supplements initially and stopped when i no longer needed it. I felt tired on almost every run at that point.
More recently
When I was training/qualified for Boston, I was tired a lot and had the same kind of lethargy as when I was anemic but my blood tests came back normal and I attributed it to a bit of over training (had about 3 doctors visits in that period for my lethargy/mood). Raced Boston and continued to feel like crap for another year(moody, couldn't finish 30 minute runs, etc). All of the sudden I developed Bell's Palsy (half your face goes numb) and luckily the doctor I got tested me for Lyme's as that is a sign of a long infection with it. Sure enough, I tested positive. Of note, I don't remember any tick bites, I don't live in an area traditionally assumed to have Lymes, and I never had a rash I could see. When I contracted it who knows but the infectious disease specialist I saw said that my history suggested a long term infection. Regardless, the routine treatment worked and I am not affected any more.
So, I would highly recommend you check this out.