Member logged in as Admin?????

One of my members logged into the forum about 30mins ago to find it logged them in as my username.

Now I don't know this member on a personal level and he lives about 100mile away. he does not have access to any of my passwords. this was a random thing that happened.

I've changed my password to see if that will cure the issue

Site - http://staffs-vw.forumotion.com/forum.htm

Anyone else experience this on their forums?

somehow I think the user did not guess an 18 digit password which has various letters and numbers in it...