“Fibro Fog” is a well-known term to describe the cognitive impairment that comes with fibromyalgia. One day you have a very active, able brain; sharp, focused. The next, you can’t remember the word for “pen”.
It’s a very common symptom and losing the ability to recall common words is very upsetting. It doesn’t happen all the time, or with the same word. It just strikes at inopportune moments. Suddenly, words with which you are very familiar and that you use every day seem to have fallen out of your head.