Posted 3/21/2017 9:05 AM (GMT -5)
As far as I know, the only way to really tell is to put yourself (carefully) in those positions again. Make sure you replicated the 'conditions' - when you ate, how long you have been up and moving around and such. Try standing for that long again - does it happen then? Try sitting down and talking about dogs (or other happy subjects).
If it's POTS, it will likely happen no matter how things change, but the other things may be able to be replicated, and you might be able to figure out what it is.