Just changed into a new password like three months ago, and it keep ask me to change into a new password, and the system is very stupid in denying that my new password is correct and constantly ask me to set up a new one after another new one. Is my iphone hacked?
This type of thing sounds to me like something somewhere has an old password stored someplace and is trying to use that password to log in resulting in the account being locked. This could be iTunes on a computer, iCloud or even your web browser or password manager if you are trying to log into an Apple related web service. It could even be some other iDevice like an iPad or iPod. If that is the case you'd have to locate what ever it is that isn't using your new password and make sure to change it there the next time you change your password.