It may not be your account that was hacked. Naughty people on the net send out jokes and funny pictures, spam warnings and so to onto hundreds of computer generated email addresses, hiding their own email address in the "to:" string. People have a habit of sending these on without stripping off the previous senders/receivers which get sent back to the originator (naughty person) along with all the email addresses of the real people who sent and received the email. Similarly the person who got hacked (not you) had your email address in their contacts and that's how the hacker got hold of it and is using it to send out the dodgy emails.
When my bit of the NHS got NHS mail, there were people using it who had never used email before. They saw no harm in forwarding on jokes, warnings and so on and it took about a month for all the inboxes to get stuffed with so much old junk that it froze the local server. It dept wised up and put in spam filters and sent round a whole list of "DO NOT"'s and the problem subsided again. If you are going to send on jokes and so on, copy paste them into a new email and blind cc them to your friends but honestly better not to do it.