Comes on when ‘slogging’ it out i.e a heavy going muddy cx race and today fighting strong headwinds. It disapears as soon as the conditions change i.e hit firm ground or turn out of the wind.
It’s nothing to do with time in the saddle.

It’s a band of dull aching across my lower back. When it happens it’s hard to keep laying power, I can but it’s very distracting.

Is this a classic case of core strength weakness?