I don't think that the collapsing has anything to do with your HT, it's just because you got up quickly and rushed to the bathroom. I remember a year ago or so I had been for a run and was sitting down in the living room some 30 mins later, my wife asked me to open the curtains so I got up quickly kneeled on the arm of the sofa to open them and before I could open them I felt very dizzy and fell back, luckily I was on the sofa and just landed on the sofa, so didn't get hurt.
My blood pressure is perfect by the way and I'm am very fit.