There is some sort of trick if I remember well...something like singing the words to a simple song (happy birthday for exaple...min 23 secs.) when changing positions...you have to move slowly and concentrate on first lowering your feet to the floor, then raising your body and then finally working to the standing position...the most dangerous changes in position are standing up after sitting on the toilet (we tend to think that because it hasn't been that long we can just pop up) and getting out of the bath (not only have you been sitting for a while but in hot water! and we are coming up from a lower level. You need to move yourself slowly onto your knees then sit on the edge of the tub and finally swing those legs over the side...then stand up really slowly while holding onto something)...keeping a stable chair at the side of the tub could help too, it gives you something to slide onto and a sort of handle to stablise yourself...
As to the causes it can be anything from dehydration as Kathy said to low blood pressure on through to a dzn other causes...the first order for me is to make yourself safe and then call your doctor for bloodwork and an apt...
Sharon
ps..how is your salt intake?