Default [Q] Phone keeps Encrypting and have to format.

Hello,

I have been running latest CM for a while but recently, on several occasions, the phone sticks on Logo, and when I reboot in recovery it asks for encryption password. I never encrypted phone, and during these instances the internal storage shows 0Mb. I have to Wipe completely and reinstall. Then the same thing will happen on reboot. What could be the cause?

Sorry, I am fairly n00bish at this so bear with me

Nick