Idiopathic hypertension (hypertension with no identifiable cause) is very common (95% of all cases of hypertension).
In your case you could call it bad luck, unless there are reasons for your hypertension.
Reasons could be stress, too much intake of salt or liquorice sweets, being overweight etc.
To get a good idea of your average bp, you should repeat the measurement several times during a period of a few days/weeks and take the average of the values.
With lifestyle changes, you can perhaps get the bp within normal range, otherwise you probably need medication to prevent cardiovascular diseases in the long run.