Unfortunately cognitive impairment can be a symptom of CFS. I have it too. Yesterday in front of a bunch of people at work, I forgot how to use the copy machine!!! So embarrasing. I have fibromyalgia too and it's called "fibro fog". I knew I had trouble when I forgot which was east and west were (always having been very good at directions) and I get my right and left mixed up. I forget what the word for something simple is, and use "thingy" a lot to describe things. I am glad your doctor is checking to make sure there is nothing else causing the problem. Hang in there! You aren't crazy and we all arent stupid! I hope you find answers and solutions.
Dx 2002 Fibromyalgia
Also have RLS, UARS (like sleep apnea), hypoglycemia and arthritis (neck and knees).
Dx 2012 with Chronic Fatigue Syndrome.
Trying to stay positive and live one day at a time.