Peaks/Valleys/luck. Honestly probably overdoing it or just very unlucky. But ultimately maybe the body just needs rest to get right back at it. I don't think your body is breaking down. You may just be overtraining. I make the mistake all the time. I think a few of us have talked about it but that insatiable need to workout is sometimes just as bad as any other thing you can do. I'd say rest, but any time I tell myself to rest I cannot stand it. We are all just built that way.