iPhone 7 Plus battery problems

I’m having a issue all of a sudden with my iPhone 7 Plus which started about 3 days ago. I notice my phone was draining with every minute even in stand by. Example went from 100-82 in 15 minutes with it being in standby for most of that time. I closed all apps, I performed what I assume is a soft reset holding the power button an the volume buttons at which it restarted and the battery was red then blinked and went back to the 80% an continued to drain so I did this two more times with the same result end up doing a complete factory restore and restored from back up to the same drain so I tried one more time with the soft reset with finally the phone restarting and the battery was not red but back to the percentage of charge. Woke up this morning again the battery is draining in standby from 100% to now 85% while not being used as I’ve been using my iPad. Is this a battery problem? I’ve turned all notifications off and app refresh but then what’s the point of having these features If I have turn them off. Has anyone else experienced this and how did you correct it. I’m a little up set after just paying off my $850 phone and just went out of my factory warranty.