Similar deal with me. I've only went down once and sat down quick on the floor. had football practice like 3 weeks ago and was on the couch watching sports. stood up quick, took two or three steps, then sat down real quick. mrs. dave went into panic mode and still has a complex that I'm dying (I am, we all are, but not from this wife get it together).
Have you checked your blood pressure? This sounds like high BP. My mom had it in her late years and I was always helping her get up and being ready to catch her from falling. This could be serious; might want to see your Dr.