Like others have mentioned, you can change your email password. however, these things happening does not mean it is in fact coming from your email account. With all these spammers/scammers and technology, they can actually setup a ghost email that when it reaches the recipient, looks like it came from your email address but in fact it came frmo somewhere else. I learned about this from a customer who had it happen to her and they were worried about their computers being hacked since it was their business email address.
definitely let your provider know so that you don't get shut down for spamming. that happened to my brother--enough complaints were made that AOL shut him down and required him to clean his computer before they would set him back up.