I think I'm getting a handle on the symptoms I have when blood lactate is elevated. Basically it's a nauseated/poisoned feeling, similar to motion sickness without the motion. So a brain-based nausea, with no real stomach involvement. Trying to do much cognitively makes the nausea worse. A little bid of headache pain, mostly from thinking, but not what I'd consider a proper headache yet.
It also seems to come after too much exertion much earlier in the day, so far about 3-6 hours afterward. So maybe it's in that period between over-exertion and the start of a proper crash. Or maybe it's occurring even if I don't progress to a crash.