Sounds like your E-mail account has been compromised, or you picked up a virus.
Immediately have your computer checked for a 'bug' or 'bugs.' If you have the proper programs, do the scans yourself, if not you need to take your computer to some one who can.
Once it is clean of pests, install a firewall and an anti-virus program if you do not have one yet.
I strongly recommend a program for malware and adware also.
After you finish this, then go in and change your password, using a good strong one.
Also, check your account information and make sure no information was changed, if it was change it back immediately.
Then, you will have to inform your contacts that it was not you sending that mail, but they need to check their computers in case they were also infected.