Something could have gone wrong during your latest update.
I would install a fresh copy of iOS on your 11, creating a full backup of your device, then restoring it to factory settings and, in the end, setting it up as new. To perform that procedure, follow these instructions:
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
When done, test your device for a while, WITHOUT restoring data from backup. If everything is okay, then restore data from your previously made backup:--> Restore your iPhone, iPad, or iPod touch from a backup
Regards
Giulio