You are absolutely right. BP changes very rapidly according to circumstances. Having said that, there are accepted 'rules' to observe, before taking your blood pressure, for a reliable reading.
For example, relax and be seated on a firm chair with your feet flat on the floor and your arm resting on a table so it's level with your heart. Not to measure right after a meal, consuming alcohol, or after a heavy workout. You should make sure you're resting and not feeling anxious or stressed.
Ensure you purchase a reliable BP meter that is clinically validated. By regularly monitoring your BP I think you can avoid sudden unpleasant surprises IMHO.
Speak to a medical professional, he/she will explain things to you very much better than what I have written here.
Measure Your Blood Pressure
