Are you using an application to set/unset your PIN?
I had this happening to me repeatedly, however it was accepting my PIN as the password so was more just annoying that anything else.
After some experimenting I found that it was Secure Settings (setting the PIN) that seemed to be the cause - somehow it was setting the PIN correctly the first time it ran and then subsequently it would switch to password.
I ditched it and instead setup my set/unset PIN via Pebble Locker (also using Wifi and other bluetooth devices to set/unset PIN).
After using it for a few days successfully, it locked once setting a password instead of PIN, and this time my usual PIN failed to unlock it. Luckily it was about 11pm and I knew that it would automatically unlock at about 7am the next day when the wifi was automatically switched back on - so I didn't need to reset or anything (although I got the impression at the time that you could unlock it via going to your google account somehow - but I can't recall the details and may have this entirely wrong).
I setup a Tasker profile so that I'd have a backdoor to disable the PIN by plugging it into power and checking if I'm at home at the time. Not the most secure solution, but hardly wide open either.