This is a complex issue which cannot be diagnosed over an Internet forum. It is recommended that you see your doctor who can take a more detailed history, perform a physical exam, and order lab tests, including but not limited to complete blood count, chemistry panel, and thyroid function tests +/- specialized blood testing (e.g., ferritin, iron, vitamin B12, and folate levels). This article provides an overview of fatigue: https://www.aafp.org/afp/2008/1115/p1173.html
Hi, Magmil. I see from your posting history that you've been to a doctor. You also probably know that all top vegan docs from Barnard to Greger recommend taking B12. You might also know that some Paleos and Carnivores say that Veganism harmed their health, just as there are Vegans who say the exact opposite (e.g., McDougall had an early stroke).
Yep, I know that you are vegetarian and not vegan. But the idea is that some diets are not for some people, to varying degree.
Otherwise, since you are low BMI: do you also exercise a lot? I'd think about Female Athlete Triad (FAT), which is well known to cause amenorrhea. The amenorrhea is not as nebulous as the fatigue, and so it's probably more productive to zero in on that. Maybe once the amenorrhea is resolved, the fatigue will also disappear. FAT also can explain the low energy.
The long term worry in FAT would be about bone density.
Btw, I've seen women say that they take months to recover from long term FAT. Getting more cholesterol wouldn't hurt in having proper levels of female hormones, either, since they are made from cholesterol.
[I am not at all an advocate or critic of any particular diet.]