Preventing unauthorized access to your message board

I received an email from a moderator regarding my message board in the campus which I often used during my college days. I don’t remember any instance in which I used this board in public or in which I logged on to it carelessly without considering the place where I was. I am confused as to where the moderator got this message board. Could it be possible that he accessed this from my IP address? The software/hardware I am using is Google Chrome, Mac OS X 10.7.2. How can I protect myself and how will I know who this moderator is? Please help me solve this issue and to stop or prevent this from happening again. Thank you so much.
