This could be a message from your browser. I read that Google Chrome now warns you if the password you are using for a certain website has been leaked online somewhere in a previous hack. This is to warn you about this and to encourage you to change it.
More information can be found in this excellent article by The Verge: https://www.theverge.com/2019/12/10/21004434/google-chrome-79-password-protections-security-stolen-password-data-features
Many password managers are available these days, which you can use to generate a unique password for every website. Many of these apps are free or built in to a browser or operating system.