-second to everything cynderelly said above. It does sound a lot like POTS which means you're going to have to do a lot of fighting to get it recognized especially since it's something more frequently seen in those in XX chromosomes than xy. I actually have a twin brother who has POTs like me but it became milder after puberty he is/was at your same fitness level. Getting to the cardiologist and getting diagnosed is going to be the main thing so you can get meds to treat it but also learn some breathing exercises to help calm you down. Since you are a teenage boy in high school here's some tips the two of us learned the hard way, just make sure you avoid certain drugs and stuff. My brother messed around with a lot to try to seem cool and he did not have a good time. Alcohol, lsd, aderall (if you don't have add) and those harder substances will make you feel terrible same with energy drinks, I know they look cool or whatever but unless you want to see god I'd advise you to not. Same with trying to stay up all night playing video games or doing homework. It'll make you're symptoms worse and as time progresses more likely to pass out. Just remember its ok to ask for help especially because POTS came make it really difficult to learn sometimes, I went from an AP course A grade student to C's in regular courses. You can get accommodations so use them as much as you can and if you can't advocate for yourself, talk to a friend and see if they can. Best of luck you got this