I haven't drank kava yet myself but .. Do you drink heavy or heady kavas? Maybe it's something so do with the stimulant effects?- not sure if kavas heady/heavy effects are anything like cannabis or not but there have been times where i could feel my blood pressure was high after smoking, depending on what strain I smoked (altho I think even heavy strains can affect your bp) and the state of my mind and body. But I even find things like chamomile and lemon balm to have quite a strong effect on how my body feels, I've had nauseating effects from all of the above and I find nausea can affects my bp.
The only thing I can think of to help is, other than seeing a doctor is, diaphragmatic breathing (and slowing down your breathing as well) and seeing if there are any factors in your daily life that could be contributing to high BP.