Monitor your calories. Sometimes we’re off when only estimating. Make sure not to skip meals. And maybe you like some nuts and other additional stuff. And probably also work out and add some more weight in muscles.
The internet contains instructions for people who like to gain weight. It’s not so easy to change your body weight. But it’s possible.
If in doubt, ask your doctor. You can be born with this and it’s perfectly normal for you. There can also be something wrong with the thyroid. Or a tumor. Other less severe conditions. But most likely this is just how you are. Especially if you’re young.