My bp is usually 110/90 or 120/90 .. the diastolic number is always around 90 or 85, i'm 18 year old female. is it prehypertion?

At any age 90 is too: high, associated with serious long-term effects. At your age the risk is higher. intervention needed to keep diastolic below 80. Hypertension evaluation indicated. Was the cuff too narrow? Do you have a stricture of a renal artery (which should be checked with ultrasound)? Family history? Have you become overweight? now is the time for answers. You need a PCP for continuity of care.