For most patients with chronic fatigue syndrome, even a small amount of exertion can be too much for them to bear. This is due to a condition known as the post-exertional malaise, the defining characteristic of the illness. Meanwhile, patients with fibromyalgia do not have post-exertional malaise however some of them possess a similar type of intolerance for exercise.
Some of these patients may find that showering takes more energy than they thought. The act will require them to stand the entire time, which is difficult for them. Plus, it will involve some stretching, bending, and lathering of the entire body, which makes the situation even more difficult.
In fact, most of these patients will find it difficult to start a new exercise routine that involves repeating certain movements, such as performing yoga. So it makes sense why they find showering too much to handle.