I tend to low blood pressure too - I think low 90 something over 56 was the lowest I ever saw measured.... I find hydration can affect it a lot. If I make sure I'm drinking enough, and by enough I mean a lot, the head rushes are kept at a minimum. Try drinking more and getting some electrolytes with what ever liquid you are using. Thankfully I've never actually passed out or fallen but I have had my knees buckle and everything starts to go grey for a while sometimes.