You may or may not be naturally thin.
But I was 160cm at college, and when I was very ill with IBS, my weight dropped to 50kg. That was still technically within the healthy range but I looked very thin. I was physically cold all the time. If I had been 4 inches taller and half a stone lighter I would have been incredibly thin. I would consider your weight in line with the fact that quite a few researchers think 18 is too low for a healthy BMI, and it should be 20. That would mean an increase to 58kg, and might give you an idea of why am "only a bit underweight" BMI is potentially impacting your health.
I've somehow had a big collection of friends with eating disorders over the years. One friend I used to talk to her and suggest maybe her portion sizes were a little out, but she used to genuinely believe they were reasonable. She also used to serve others that portion size, so yes, she ate the same as them, but they would also have pudding where she didn't, and they would snacks and she wouldn't. Breakfast would be half or a quarter of the size, and was porridge (notionally good) but made with water and with raisins on top (again notionally good), but in fact it can't have totaled more than 150 calories. The same would apply at lunch time. So something notionally reasonable - like say a ham and tomato sandwich. Except it would be one of those little loaves of bread, so around ,55 calories a slice (110 total) plus ham and tomato (no butter) at about 30 calories. (Total 140) She would then also have an apple (50) and a big glass of water.
So what she had if you described it was a perfectly reasonable "porridge with raisins and then a sandwich and an apple for lunch", but what she ate, when you examined the details, was a miniscule 340 calories.
Other friends used to cook dinners, and it would be the same dinner for everyone but it might be very likely to be say, a chicken breast salad, with butternut squash and salad leaves and lovely fresh herbs and maybe balsamic vinegar. No nuts, no fatty meat, no bread served with it (or maybe they do serve bread but they don't eat it, or its a baguette cut into small pieces and then have just one). It's a lively, vibrant, healthy meal . It's the sort of meal most of us feel we should be making regularly, and we should. But the cup of butternut squash is about 80 calories. Maybe 200 calories in a chicken breast. The salad will be negligible but let's say 100 calories of oil for cooking the butternut squash and for the balsamic. So the whole meal is 400 calories. Everyone who has a good amount of bread and butter with it then consumes another 300 on top. If you combine that calorie intake at dinner with a decent chunk of exercise and some not terribly high calorie breakfasts and lunches you soon get to a point where you "all ate the same at dinner" but you're actually eating unhealthily few calories.
I'm not saying this to tell you that you do or don't have an eating disorder. I'm more pointing out that calories can be deceiving.
I still haven't got the balance right myself. I don't enjoy calorie counting because all it does is make me think of all the friends I have watched, consumed by eating disorders. So I don't, and I'm a bit overweight. But I am healthy, I walk miles s day, swim miles a week, and my IBS is under control. I'll take that, for me.