I am an idiot. What a complete tube.
So the thing I was expecting to arrive still hasn't arrived - and then it dawned on me. Last year I got the coil fitted. The kind I got makes your periods lighter. In fact it makes them so light that they often fade away to nothing.
Yup, the answer was that simple. And the reason I was so tired was because that was the time I would have had my period. Thank goodness I didn't get all worried and convince myself there was a bun in the oven.