Assumung this is an average reading and not just a one off reading, then yes you have mild hypertention (pre-hypertention in the US). 145/90 isn't indicative for drug therapy unless you are in the 'at risk' group.
Normal healthy, optimal BP is <120/80. In the vast majority of cases, the cause of hypertension is unknown. This is know as 'essential hypertension'. Secondary hypertension
can be caused by other problems- a quick check up with your GP will exclude any other underlying problems.
All you can do is continue taking regular exercise, reduce sodium in your diet and stay as close you your ideal weight as possible. See your GP.