If your internet disconnected today then that's almost certainly the cause - the old AIM session wasn't signed out of successfully so they think you're trying to log on from a second location. I also get this all the time when I to sign on to aimPro without signing out of the normal aim client.

Does anyone know if they show you the connected IP's if you hit "2"? If so, do that next time and see if there's a connection from another ip than yours (if your internet was disconnected then the ip might have changed, but it should be similar up until probably the last 3 digits. If it isn't remotely close then someone is logged on to your account from elsewhere and you should get ultra paranoid and wipe everything then change passwords.

Oh and btw- you obviously shouldnt use the aim password anywhere else, so even if your account is compromised it's not a huge worry. Still reformat and change the other passwords just in case, though.