I have polycythemia Vera and I am getting phlebotomy to treat. During my last session the nurses kept having to flush the iv due to clotting in the tube. After about the 10th time I felt very nauseous and thought I might vomit. That's all I remember before waking up to nurses reclining my seat and asking me if I'm OK. I had a BP of 45/15. I've had dozens of phlebotomies before this one with no issues. What could have happened to cause such a drop in BP?