Thanks for posting your query.
Your doctor is right you do not need to worry as 120/90 is normal or borderline blood pressure. Systolic blood pressure more than 139 and diastolic more than 90 is considered to be high.However, single reading of BP is not enough to diagnose, so you should check your BP regularly.
You need to elaborate what type of upper body discomfort you are experiencing, it may be due to muscular activities.
You can prevent high blood pressure
to develop by regular exercise, healthy diet, low salt intake and reducing stress
Hope it helps,