Low blood pressure with high heart rate can be from autonomic system dysfunction/ P.O.T.S. Sometimes a lot of symptoms can intensify as a way of healing paradox. Not 100% sure what is going on though. Perhaps the added potassium needs to be balanced with more magnesium, and sodium. Eating more sea salt will at least help bring the blood pressure back to better ranges. Try taking Transdermal Magnesium, keeping supplemented potassium between 1000-3000+mg a day, and adding more sodium in the form of sea salt. Potassium levels whether spiked lower or higher can also cause changes in nerve impulse, and heart rate. Get doctors approval first of course, none of us are doctors nor can condone medical advice.
(I say that last part all the time just because of legal reasons lol)
P.S. the RDA for Potassium is 4000mg a day. Magnesium is 400mg, 2300mg for sodium. Magnesium can only be absorbed and utilized so much at once, I often recommend trans dermal becuase it is much more bio available and immediate. You also absorb more then what is lost in the GI system.