Had the exact same thing happen when I was doing a Ketamine therapy thing a while back, and I definitely think the anxiousness about the reading had something to do with it.

I've always had perfectly okay blood pressure whenever I have it done at a regular doctor's appointment, so I think knowing that the reading actually matters definitely increases my blood pressure. Ended up getting a doctor's note saying "they don't actually have high blood pressure they are just reacting normally to possibly having to be sent home and reschedule" more or less.