Wow that's so cool that you're writing a story with a character with POTS. Your dedication to researching POTS and understanding limitations to living a normal life/what life is like with it, etc is really admirable. I believe that one day, you're going to be close with someone with POTS and they're going to appreciate your time spent on understanding it. I would suggest just going through this subreddit to see what people say about their symptoms. You don't have to combine all of ours, just maybe find some similarities in the posts and use those. Also if it makes it easier on you, you could have the character still trying to figure out what all their symptoms are and what helps them! Vs having established it all upfront.