I used to pass out regularly if I didn't immediately sit down when I felt the same thing you're describing. I told my Rheumy and her and I actually giggle about it because she has the same issue --- it's a rapid drop in already low blood pressure. We giggle because one day she stood up to kiss her husband and right after he kissed her she fainted so he was like "WOW that was some kiss!"
I don't have it as much now since my blood pressure has stabilized (still kinda low but not as low as it was) but I still get the headrush every now and then.
With some people it's their low blood pressure and with lupies I think it warrants further investigation. I also have Raynaud's and my rheumy just recently tested me for anticardiolipins (sp?) which I think shows if your condition is affecting your blood.
Whenever I don't feel like doing it, I always remember that tomorrow I may not be able to do it. Then doing it becomes a blessing instead of a burden.
I got some stuff goin' on.