It’s called white coast hypertension, I don’t think of myself as particularly nervous but I get this too. I’ve had blood pressure readings in various medicals etc. from 145-160/90, and endured several stern lectures about salt and exercise and alcohol. Finally did a 24 hour ambulatory test and I’m < 130/60 or something in a non-clinical setting.

After this experience I did some reading and as far as I can tell, blood pressure is the least consistent, least rigorous test you can do in mainstream medicine. The “normal” range spans ~10mmHg and yet readings can vary by 10-20mmHg according to each of: stress level, standing up in the last 15 mins, recently having a drink of water (as you do in a doctor’s waiting room), whether your legs are crossed, etc. Without controlling for all of these (which is almost never done properly) they might as well be reading your tea leaves.