hello i am 17 years old, male, about 140 pounds, skinny, and have this weird thing going on, ok so whenever i stand up my heart beats hard and at a low rate but not life threatening rate maybe like to 55 or 60 then it speeds up really quickly, when im having this thing, i feel kind of out of breath but i can breath fine but it feels like i have to breath a little harder, and rarely it happens whenever im sitting down. also when im taking a hot shower my heart races. i tryed to look this up all i found simular to this was orthostatic hypotension, because it feels like im going to black out, but it onlys happens for about 5 seconds then my heart races goes to normal after that please can anyone tell me what do i have, also i have not fainted or blacked out before, my family and i have gone through 10 tests on my heart and went to a cardiologist to take a cardiogram, to take pictures of my heart, and they said i was fine, but dont know what going on right now, so if anyone can help me with this or if i can get treatment for this?

Summarized question: What are the symptoms of orthostatic hypotension?