you could always try soft-hard reset, reflashing 8.1 and upgrading again, or lazily wait for a new cumulative update
if you "upgraded" recently your phone might be busy, I mean, if you have enabled auto photo upload to onedrive and have a ton of them, its probably trying to upload them all at once right now, that can lead to all kinds of erratic behavior, maybe you have that skype integration enabled? it will kill your cpu-battery faster than you could imagine while it syncs all your conversations from 2006, try turning on battery saver and see if you can use your camera that way
kind of sad how this could potentially be fixed by a simple app reinstall, but of course Microsoft wont let you uninstall that app