it's not POTS??

i have a pretty low resting heart rate, typically in the low 40s, but standing up can get it up to the 140-160 range, and showering can easily get my heart rate into the 170s. i've been having this problem for over a decade (i'm 24 now) and have passed out many times. usually my vision just goes out and i can get myself to the floor until i'm feeling better, but not always, and i've passed out in some unfortunate places.

i've also got the usual dizziness, sweating, always tired, and poor circulation.

after wearing a heart monitor for a few days to confirm this my doctor sent me for a tilt table test. they said it's not pots. my doctor gave me salt pills to try anyway and they've been helping. does anyone know what this could be if not pots?

Edit: thanks for your opinions everyone. this was helpful to get some insight. i probably won't be able to get a second test done or second opinion for a while since the first time around was over $2000 (thanks insurance companies!) but i'll hopefully be looking into some options in the future