A stress fracture is lots of small loads with no rest. Most burnout is built the same way.

Bone does not break here from one big hit. It fails from repetition without recovery, each load too small to notice at the time. That is why the injury seems to arrive out of nowhere when it was being assembled for weeks. Exhaustion in the rest of life is built identically: nothing individually unreasonable, no gap anywhere, then a collapse that looks inexplicable.

Check whether there is a gap anywhere in your week. An evening with nothing in it, a morning without an alarm. If you cannot find one, add it before something else makes that decision for you.