There's a couple of things that could be going on here. First, look in your Sent folder. Do you see anything in there you know you didn't send out yourself? If so, then yes, your account has likely been compromised. In that case, change your password to something stronger than whatever it is right now. Also, change the secret question and answer for the account. Then run a complete scan of your machine with your A/V software to see if it finds anything wrong.
The other likely scenario is that your email address is being "spoofed". Spammers can make it appear as though emails come from you when in fact they did not. If that is the case, then there is nothing to do but wait it out. Spammers will usually use an address for a few days or a week and then move on to annoy someone else.
In either case I would change the password right away. Most of the time that will stop the problem if the account has been hacked.