BitLocker, and most full disk encryption systems, do not directly use your password for encryption. Instead, your password is used to encrypt a random, computer-generated master key which is used to encrypt data, and the encrypted version of the master key is stored on the hard drive. When you unlock the drive, you decrypt the stored master key which is then able to encrypt and decrypt the rest of the drive. This scheme makes it possible to change your password quickly without needing to re-encrypt every byte on the drive (only the comparatively small master key needs to be re-encrypted).