21 y.o. with bp 136/90 bmi 22. got same pressure 2x on 2 machines by dr. borderline prehypertension? cause? healthy otherwise. is it a fluke?

Hypertension: That pressure is right on the cusp of the definition of hypertension. At 21y/o you need an evaluation to see why the pressure is sitting there. You should be on a low salt diet and exercise program while the evaluation is being done.

Need more BP checks: You should check your pressure at the pharmacy / grocery once a wk or so over the next couple months, and record the values. Your doc will probably be happy to go over them with you. If you average around the same pressure, around 136/90, then you do indeed have pre-hypertension. Either way, you should be doing routine exercise (1/2 hr 5 x/wk), and avoiding sodium in your diet (<2 grams per day).