That was maybe a presyncope episode. I get those when it's hot or I'm in a room with lots of people - feeling hot, dizzy, nauseous, sweating, can't breath, having vertigo and tingles in my neck and back of head, starting to feel numb, everything starts to fade out and I feel like that my legs will "chop off". Afterward I'm all in pain, have a headache, shake and feel thirsty. Heat and low BP don't go well together. Since you have POTS that doesn't make things any better. Always be sure that you're well hydrated. Next time you get an episode sit down (on the ground) somewhere in shades (important since you need to cool down) and put your head between legs or lay down flat (so that your heart can pump up easier) and drink at least 200 ml of water. That will stabilize your BP and heart. WAIT for some time until you feel better (15-30 mins, depends on you). Don't get up too fast or you'll feel even more worse. I've found that for me it's wise to avoid direct sunlight because I almost instantly get tingles in my neck and start feeling dizzy and nauseous, specially if I need to stand for few minutes.