User in sudo group cannot use sudo command

Since you error message says your user is not in the sudoers file, could you please check your /etc/sudoers file, see if there is such a line:

%sudo   ALL=(ALL:ALL) ALL

If this line is missing, users in the sudo group will not be sudoers. Edit the file using visudo command (checks for correct synthax and locks the file).

You can also try to add a customized line like this:

root    ALL=(ALL:ALL) ALL

Replace root with your username, reboot, and try if it works.

As per your conversation with @steeldriver in the comments, my guess is that your user isn't logged off.

The easiest way to make sure that the supersecretuser has logged off is to type into terminal, as another user, who.

When doing so, I get output like this:

mitch@quartz:~$ who
mitch    :0           2014-09-08 09:49 (:0)
mitch    pts/0        2014-09-08 13:18 (:0.0)

I only see myself. Similarly, typing w into a terminal shows me who all is logged on and what they're doing:

USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
mitch    :0       :0               09:49   ?xdm?  29:08   0.11s init --user
mitch    pts/0    :0.0             13:18    5.00s  0.06s  0.01s w

You can STOP your other user, (SSU I'll call it) with the following command:

pkill -STOP -u SSU

However, truthfully, the easiest solution would be to just reboot the system if that's possible.