I took a pre-workout drink once, and ever since my diastolic pressure is >90. what can i do to lower it?

Doubtful: I doubt that the two occurrences are related. Are you measuring your BP at complete rest (for 15 minutes)? Is the cuff large enough for your biceps ? Many brands of birth control pills cause htn. I suspect one of the above 3 issues is contributing - not the drink.

First verify: I have no idea why you are relating your BP to a drink, pre-workout or any other time. The only blood pressure measurement that matters is the lowest one of the day. Get an automatic BP machine that uses a cuff on upper arm (not wrist or finger) and take your pressure at a quiet time and the same time (like when you wake up before eating) and write down the results for two weeks. It will be.