Hi Rosie,
Fibromyalgia (FM) and Myalgic encephalomyelitis (ME) are both conditions of exclusion - meaning that they are each a diagnosis we receive when we present a certain pattern of symptoms and no other causes can be found - and they certainly do have some overlap. Some people are diagnosed with just one of the conditions, while other people are diagnosed with both simultaneously.
Fibromyalgia's primary feature is chronic and widespread pain, with fatigue, fog, and depression serving as the "backing singers" of the show - if that makes sense!
Meanwhile, ME - which is also known as chronic fatigue syndrome (CFS) - has fatigue appearing as the unwelcome star, with pain, fog, and depression as the supporting cast.
This is a bit of an oversimplification, because both conditions can come with a range of other lesser symptoms, and can manifest with quite varied presentations - but as a general rule of thumb, fibro's primary indicator is pain, while ME's primary indicator is fatigue.
For their similarities, scientists have figured out that the two conditions are not entirely the same. Studies conducted on patients with FM and ME have found that certain chemical markers in the brain are different between those suffering from the two conditions. For example, neurotransmitter Substance P appears elevated in the cerebrospinal fluid of people with FM, but not those with ME. Unfortunately, these are not things that doctors can examine for everybody.
I hope that helps!