Why did I feel so tired? A while ago, I had a spell of feeling physically exhausted, despite not having particularly exerted myself. I wasn't ill, as far as I knew. I found the answer by reflecting on my mood.
Up to that point I'd had an extraordinarily stressful time - not especially busy, but a lot of anxiety and worry. I don't know about you, but I sometimes forget that this sort of mental pressure can literally take it all out of you, making huge demands of your physiology as it causes your systems to work overtime producing all the hormones associated with stress, and it's really not surprising that you need time to recover once things return to being on an even keel.
The learning? It's so important to listen to your body, and to look for clues such as mid-morning yawns and general lethargy. Obviously, if there's no clear explanation for this, it's sensible to seek expert advice and help, but if it's simply that you've been having a tough time (which you're thankfully now beyond) it may be a straightforward case of allowing nature to work its wonders, getting you fixed in a day or two. There's a big connection between what you do and how you feel.