There are several proposed causes of MS fatigue.
For a lot of people, every day fatigue is caused by the MS brain having to work extra hard to do things. (Sorry in advance for the rather silly analogy, but I couldn’t think of another way to explain!) If you can imagine the brain wanting to send a signal from A to B as getting a coach load of people from A to B. A normal brain would load all the people on the coach at A, head off at speed along the nice wide motorway and arrive at B without any hassle. But the lesions and axonal damage in MS means that the brain has to deal with lots of traffic jams and diversions. So getting the coach from A to B means taking longer, less well maintained routes, e.g. lots of meandering country lanes. But the coach can’t fit down the country lanes, so the MS brain also has to get areas involved that weren’t original designed to be involved - so instead of using a coach, it has to load the people onto lots of cars and bikes. In reality, this means that people with MS use far more of their brain to do things than people without MS. The normal brain uses up a massive amount of the body’s resources. The MS brain, with all that extra work, uses up far more - and it’s exhausting.
There are other causes of everyday fatigue too though. There are at least two parts of the brain that are responsible for monitoring fatigue. One is in the parietal lobe and kicks in when we are physically tired. The other is a team of areas in the frontal and limbic lobes. These kick in when we are mentally or just generally tired. Damage to either of these, or to the connections to and from them, could cause ongoing fatigue, even if there is no actual reason for the person to be fatigued.
Fatigue that hits just like a relapse is not the same as everyday fatigue. As far as I’m concerned, it is a relapse, even if nothing else is going on. That’s because it is probably a sign that the brain is under attack - that new lesions are forming. A lot of MS lesions are “clinically silent”, i.e. don’t cause observable symptoms. So if MS is causing new clinically silent lesions, we aren’t going to have new symptoms, but new roadblocks and diversions are being built so our fatigue gets much worse. When the body repairs the roadblocks a bit and the brain learns to deal with the remaining diversions, the fatigue improves again - back to the normal, everyday stuff.
Sorry, this is probably way more detail than you wanted! It is also my own explanation of various theories so there could be mistakes in there.
The simple answer to your question is that fatigue is a symptom of damage rather than a cause of damage. Rest, rest, rest, allow your body to try and repair what it can and your brain to work out new routes.
Everyday fatigue is bad. Relapse fatigue is truly terrible You have my utmost sympathy.
I hope it gets better very soon!