There are some studies that fasting or a ketogenic diet can be helpful for certain autoimmune disorders. However this is a conversation you need to have with your doctor that is managing your condition.
Being skinny and fasting is hard. Hard in the sense of eating enough. I’m male, 6’1 and 70kg with a BMI if 21 and easily lose weight. I try to do 20:4 for the health benefits. Sometimes it’s 18:6 if I struggle to eat enough in that time.
If I get lazy and only eat one meal a day for a week my girlfriend will be able to tell (and I get in trouble lol).
Do what the doctor says. Get some weight on you. And then maybe look at easing into it.
Do you observe any autoimmune diets? They’re pretty restricted but might help you more than fasting. I just read a book on one doctor’s version and thought it might be worth trying. I don’t have auto immune issues, except allergies and high inflammation levels, but have a family history of them.
The only advice I can give you is that any kind of dietary restriction is not the answer for you. I suspect that a lot of the issues you’re experiencing are caused by or exacerbated by being underweight. With that in mind, I’d recommend looking for ways to increase your caloric intake. Healthy fats and calorie dense foods will help a lot. For gut, there’s several natural and supplemental probiotics you can try if you haven’t already. Yogurts and powders come to mind.