Flashing up from 5.0 to 5.1 with CM12 nightlies?
So, I just went through this. I have the LG G3 D851. Not realizing that it went from CM12.0 to CM12.1. You WILL need to get the latest GAPPS. I did not, and ended up hosing my data causing me to have to wipe and start fresh. 

I am now running CM12.1 with GApps for 5.1 and starting the process of logging back in to everything and recover all of my 2-factor authentication. Hopefully this helps you, so you do not have to go through the same.