Greetings, HaneenM.
It appears that your iPhone 6s has started to unexpectedly shut down while you're using it. You've also noticed a discrepancy with the battery level when it does, and I'm happy to help.
iPhone 6s Program for Unexpected Shutdown Issues -- See if your iPhone 6s qualifies for this program.
If you see "Your iPhone battery may need to be serviced" -- An iPhone shutting down unexpectedly sometimes indicates an issue with the battery. Tap Settings > Battery to see if this notice appears.
How to back up your iPhone, iPad, and iPod touch -- If your iPhone didn't qualify for the program, and there wasn't a notice about the battery, please be sure to have a fresh backup to keep your personal information safe. In order to back up your Health data, the backup needs to be encrypted. iCloud does this automatically, but iTunes doesn't. If you'd like to use iTunes to back up your iPhone, refer to this article: About encrypted backups in iTunes
If you can't update or restore your iPhone, iPad, or iPod touch -- Even though the title of the article isn't specific to your issue, the section titled "Put your device in recovery mode and set it up again" will walk you through all the steps to reinstall the iOS software on your device. Test to see if your iPhone shuts down unexpectedly again.
Let me know how that goes, and take care!