Iphone 6s gap in battery 100% to 10% in seconds!

I keep my iphone 6s on the charger every night so it is fully charged when I wake up. I have battery issues like everyone- losing charge when not in use, being fully charged and 3 hours later dead, no big deal, I’m used to it. BUT this morning when I took it off the charger and went to check on the usual, my phone turned off by itself. I figured it must be because I’ve procrastinated the new update (13.5.1) but nope- it was truly dead! It went from 100%-10% in SECONDS! What? Why is that?