ok there're many precautions you should take to make a creditable BP reading
1- you shouldn't be smoking or drinking coffee or tea for 2 hours
2- no vigorous exercise before the exercise
3- fixed time of the day

after you get 3 readings a week you get the mean of these reading
if it's more than 140/90
you should consider lowering your blood pressure
avoid eating salts and exercise alot

try this and tell me the results