Do you have your gmail setup in an email program?
Happened to one of the email addresses on our company domain as well.
I had set default passwords, and sent out instructions on how the password should be immediately changed.
Seems one of the directors decided to ignore the email, only to be flooded.
Changed the password, and deleted all offending mail. Been fine since then.
First thing is to make sure that you have no malware on your PC.
Secondly as has been said above implement the SPF
Thirdly change your password to a complex one and keep changing it regularly.
All these should reduce the incidence of the problem for you.
Yip, change your password.. I know its strange, but it works.
I agree. Change your ADSL login password as well as your email password. Most of the time these are the same, and yours may have been compromised. Someone suggested this to me and I laughed it off at first. Decided to try it, and the messages stopped instantly.