It might not actually be sent from your account just made to look like it has been. It's incredibly easy to fake the address that an email claims to be from, there's absolutely no checking or guarantee that the address an email claims to be from is who sent it.
How would they get your address book? Maybe you've replied to one of those silly "if you forward this to X people then Y will/will not happen".
However act as if your account has been hacked.
Change your password immediately.
Then, presuming you use a Windows PC, make sure that your anti-virus and anti-malware (two different things) are both up to date and run a full scan.