You want to check several times a day at the same time for at least a few days. Taking your BP one time randomly means nothing unless its way off the charts in either direction. For example, when you first wake up, then at noon, early evening, and before bed. Do the same things at the same times and youll get a baseline of where you actually stand. Just make sure you do everything the same so if you check at noon before eating lunch then always check before eating lunch. If you check in the evening before the dog walk then always check before the dog walk.
It doesnt have to be 4 times but at least morning and evening. Write it down. If you want, track your food too, it may show a trend.