Are you using some external proxy? If so, many of those get detected and banned by many sites.
The other possibility is you have some malware on your PC, and once you log in for the first time, that virus software starts sending tons of messages, using your PC and your account on the forum.
Most likely you are using some proxy. If you have no knowledge of setting a proxy, then it is 99% possible some virus has set it in the background, to send all your internet traffic via some hacker's proxy server. They now have all your user ids and passwords if that's the case, including the forum you were on. You can be sure they are spamming it right now....
Remove any proxy settings in your browsers, and try a new user account, see if it still happens.
Make another? (: