Nothing to worry about OP. Well, maybe a bit, but not anywhere near the amount you seem to be.
As said, the attackers will have gained your credentials/part of your credentials from another organisation that got hacked. When your passwords are stored, they are stored as hashes, which are a mathematical representation (called a hash) of your password put through a one-way mangling process called hashing. This produces a hash that is stored, and is compared to the hash of the password provided whenever you log in. If they're the same, you get to log in. What happens is that the database that these hashes are stored in becomes compromised by some nasty hacker types, who then use a very hardware intensive process to run through all the potential passwords, essentially guessing it eventually. This can take a long time if your password is long/complex, so sometimes it is be months before an attacker can process the entire database and then sell or publish the plain-text credentials.
Too long; didn't read:- hackers got your password off a hacked site, change your passwords and you'll be alright.