Hi.. Any BP recording where the systolic reading is above 140mmHg and the diastolic reading above 90mmHg are in the hypertensive range.. However a single reading of BP is generally not considered as hypertension.. It is the persistence of these reading inspite of complete bed rest, hypertension is considered.. But since you have a factor (single kidney) due to which you would be predisposed to developing hypertension and your BP is in the higher limit, antihypertensive small dose of antihypertensive medications can be used..
The time of action of antihypertensive medications depend on the medications used, the rate of absorption and other psychogenic factors such as anxiety too.. So, yes, your readings may have come down due to the combined effects of the medications and the affects of the psychogenic factors..