Happened the other evening and last night as well. Not that it worries me
Next time you login, check down the bottom of the page and see how many users are online.
Eg, Most Online Ever: 245 (April 22, 2012, 08:31:18 PM), most evenings there are about 200+ members online, got no idea how many guests at that time.
ATM(7.16am) there are 63 Guests, 22 Users online, so if there are 200 members online of an evening, there would be a fair lot of Guests as well.
I guess it's a numbers game