PostgreSQL doesn't randomly change your password.
Someone else is logging in and changing your password. Since your password is about the worst possible password, this would not be hard to do for anyone who can reach your box and can get past pg_hba.conf. Depending on your firewall and your pg_hba.conf, this could be anyone with internet access. Which is a lot of people.
Your quoted error message is incomplete. Did the message start with "password" which you stripped off? You can look in the server side log file to get more details on the failure than are sent to the unauthenticated client. For example:
FATAL: password authentication failed for user "postgres"
DETAIL: User "postgres" has no password assigned.
FATAL: password authentication failed for user "jjanes"
DETAIL: Password does not match for user "jjanes".