My blood pressure increase like prehypertension then its low again for 1 or 2 weeks then again like pre hypertension?

Pre-hypertension.: Numbers that fluctuate in this range could be related to your dietary intake, activity level, exercise schedule, stress level and sleep pattern. Generally, though, this does suggest some overall dysregulation of your blood pressure that should be monitored.

Common: BP numbers=: from a single heart beat systolic, another diastolic. Many BP gauges rather inaccurate. BP determined by momentary balance between heart output volume versus total-peripheral-resistance (net average size of all peripheral arterioles in body), both controlled 24/7 from brain (non-conscious mind) & powerfully influenced by both multiple feedback signals from body along with emotions (subconscious).