Don't know if it helps but I get this from time to time, for no reason at all (it would appear) my legs just say "nope, no more" and they just feel tired and very drained. It doesn't seem to matter how much sleep I get or how much caffeine I take in desperation to ride against it, the fact is theres no energy left. I can walk but I can't run- well not for very long at all.
I find it lasts a week and then I'm back to normal, seems to happen around every 6 weeks and its been happening for the past year or so, very odd but once you know its there its less worrying.
I have been told I have the beginning stages of ostoarthritis -I was told this last year after a scan on my calves, I do get niggles when I run for longer then 30 minutes at a time but these sort themselves out after by the next day. I don't think the achey legs I have and my knee and ankle niggles are linked but thats just my understanding on it. I'd use the fatigue to take things down a tone for a week, cut back by 30-50% and then go back. I don't find it helps to stop totally but it does help to do less and to have cut back- keep yourself moving.
If things get worse or you are worried you can get help from your GP but its not something they tend to do very easily or quickly and at the end of the day if it is exercise related they are just going to say "rest and cut back/stop" whatever the diagnosis is so aside from a diagnosis- which is great if like me you worry too much, the treatment isn't going to really vary that much from problem to problem.