Hi Martha, welcome to Healing Well.
Lupus fog is very common, basically it's just a feeling that something isn't clicking quite right and that you are walking around in a bit of a haze, or fog. Unfortunately, there really isn't any way to determine if these initial memory & concentration issues are from stress, Lupus, or some other cause. In some cases a course of prednisone will help with inflammation and seems to lesson some fogs.
However, with Lupus cognitive issues can also be very serious. These can actually be detected with brain scans (sorry, don't remember which techniques are used off the top of my head) and require different kinds of treatment. Fortunately, this more serious mental involvement is also fairly rare.
Read the Lupus 101 thread if you haven't already - I think there are some tips on how to cope with the fog in it. And be sure to mention to your Dr.
Lynnwood, Lupus & Sjogren's Moderator, Dx: 2002DIAGNOSING LUPUS & HW's LUPUS 101
"Life is far too important to be taken seriously." - Oscar Wilde