My DD has had fluctuating levels of fatigue all her life, has never been able to physically do what her peers could. She also has other conditions, some of which e.g. hypotonia can cause fatigue.
She's seen a lot of specialists since childhood and they can and do contradict each other about what her fatigue is. Currently she has a CFS/ME dx.
Her fatigue seems to happen spontaneously as well as when she's over-exerted herself. She's never had a good level of energy and has always had to try and pace herself, not a great way to manage it when you're so optimistic about your own capabilities hence the need for me to step in, but some days she can do as much as she can and be fine afterwards, others are just a complete write-off from getting up shattered despite not having over-exerted herself the previous day.
It also seems to go in phases, she'll be on a down phase for months where every question along the lines of what would you like to eat, or do, or what do your think, or which would you prefer, is pretty much answered with don't know and don't care. In that phase, every day is a pyjama day because she's so shattered she can only just eat and drink. Bed, bath, sofa days.
Then sometimes overnight, sometimes gradually, that changes and she's brighter and more engaged and wants to do things because she's able to do things, albeit nowhere near as much as her peers, ever.
I've kept records over the years trying to see if there's anything that triggers it, any patterns, food, exercise or anything else and nothing stands out at all.
One thing we found didn't work we Graded Exercise Therapy. it was fine for quite a while doing a little bit more every day and it gave her a sense of achievement, then with no warning, she had an almighty crash and was back to bed bath, sofa.
One thing she found a few years ago that made her feel a bit more like herself was taking a daily multivitamin and mineral supplement (Aldi's own) and extra VitD3. (BetterYou DLux 3000)
The Spoons Theory is a good way of explaining it to other people, most of whom "don't believe in it" and can be less than complementary about it.