OP back with an update, since a thread like this is useless without one.
Update, and it is the boring outcome: nothing was wrong. The measurement I was reading had moved less than its own week-to-week variation, which somebody said upthread and I did not believe.
One thing that is still open after paige_pharma’s answer:
What did you change at the same time, and can you separate the two now?
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Browse GL BiochemAdmin said:Update, and it is the boring outcome: nothing was wrong.
Correct as far as it goes. The part it does not cover is what to do when the honest answer is "not enough data", which is more often than anyone likes.