What Causes Sudden Decrease In Blood Pressure?

Svt lasted 3.5 hrs. BP during this time unormal to low several times. Finally svt of over 200 bp done and bp 85/65 Pulse dropting 105 all at resting. I m 44 yr old women. I m exhausted of course but wondering if I should go to er or make appt asap with cardio. My normal is 105/68. Give or take few . Rechecked bp 20 mins later 83/60. Pulse 99