Your username and password combo probably leaked from somewhere. It happens when services you use have a breach.
This is why it is recommended to not use the same password for everything, as well as changing your password once in a while.
Also, I recommend putting your email into haveibeenpwned.com and it will let you know if your address has been part of any breaches that it is aware of. Optional: Sign up, and you’ll be emailed if your address does appear at a later date.