iPhone 6s battery very suddenly draining quickly on iOS 13.5 AND 13.6

My iPhone 6s w/ iOS 13.5 was working fine all weekend. I plug it in to charge overnight last night, and this morning I use it for about 15 minutes and suddenly noticed the battery drained from 100% down to 42% and I could literally watch that percentage falling like a countdown. I restart the phone and it goes back up to 70% before starting to tumble again. I checked for an update, saw 13.6, installed it, but the problem persists. Now the number is being very erratic... dropping 10-15% within 5 minutes of me opening an app, then sometimes stopping at that number for another 15 minutes before tumbling again.

My max battery capacity is 86%, and the last time I replaced the battery was November 2018. The screen has no white blur and is not rising off the phone, so it doesn't seem like a bloated battery. And again, this happened very suddenly overnight and the percentage goes back up when I restart the phone before dropping again. What's the issue? Is it something I can resolve on my own because all the certified repair centers around me are booked through next Monday.