Flex your forearm and measure the size of your elbow. Is it really smaller than other people? If so, then it is likely you have a "small frame". If not, then you are just skinny.
I feel ya. Im sorta recovering for a severe episode of idiopathic anorexia whereby I was 174cm/46kg!

Im 60kg now and feeling much better, and at 174cm my BMI should be about the same as yours.
Try measuring your food intake for a week and check your daily caloric consumption.

Maintain 2500+ calories with 50g of protein a day. As a rough guide, one large egg has 10g of protein and a pack of indomee has 400 calories.

Thin people may appear to "eat as much as they like", but in reality they are actually only consuming about ~1500cal a day.
It has nothing to do with metabolism!

But rather, their satiety mechanism. They eat alot, yes. But they feel full for hours after that. On the whole, overall, in a day, they are not eating enough.
I used to think i was eating enough with two slices of toast in the morning and a big, as much as I can eat, dinner. But when push came to shove, it was only about 1500 cal a day!