As fred44nl implied, the odds are really good you've been hacked. Somebody else logged into your RPi using the default user/password, then locked you out by changing the password.
Even though it is not certain, the odds are great he has been hacked, either over the internet, or another device on his localnet. The fact that the OP is using the default password, and it keep happening over and over again after a few days is a symptom of hacking. Once a hacker discovers you are using the default password, he will try again. Many hackers are using other's RPis to bitcoin mine.Also I know the preset password is raspberry.
But two or three days later the password was changed and I didn't even change it.
So I'm reinstall my rasbian O/S and same thing keep happen agian.
Evidence?I think the key question is: Is OP's machine exposed to the Internet? Most aren't, yet all the responders so far are assuming that it is.
The command to change the password is passwd not password.
Passing -n to passwd requires a numeric value after it, it defines how often (in days) that the user must wait between changes to their password, e.g. setting it to 7 would mean once the user changes their password they must wait a week before they can change it again. Setting it to 0 (the default) means they can change it as often as they like. Not providing a value is an error and so does nothing.
Very good. I'm beginning to think that the fact that a lot of people post here from things like tablets and phones and such explains a lot of what I read here...It looks like autocorrect came into play.
Even so, there's no need to delete the current password before setting a new one. When root changes a user's password it doesn't care what their old password was.