Is the Yosemite File Vault password (usually) the same as login password?
OS X will usually attempt to sync the password if you change it, but it's possible to have them different. I have them differently right now, so that my old user password is required at the pre-boot screen, but once OS X is booted, I need to log in again with my new password.

See more info in this answer: How can I have one password for FileValue 2 (pre-boot password) and another for my user login?