Why did my iPhone's battery suddenly die at 34%?

i was on a train this morning just playing with the camera app on my phone taking photos of the outside and listening to music for nearly 10 minutes and my phone was already draining and it was at 34%, so im looking outside when my music suddenly stops playing and the screen goes black, i thought the app crashed but what i found is that my phone DIED AT 34%, it usually dies when it's at 1% and on for a few minutes, but it died at 34% and i had it on full charge at my job , i have no idea why this happened to me :( now im so worried it will do the same again, i don;t know if it had to do with a loose outlet at my job or the battery cuz my phone has never done this before and the battery is in good standing, it even died at 14% too

what do i do?