No, you shouldn't be losing, you should be gaining. But to gain weight, you should still eat healthy. Regardless of your weight, you should stay away from soda and as a rule Stay away from McDonalds because it's not good for you. It's not just about calories, its about health. You dont want heart disease. Eat healthy, just eat more of it to gain weight.

I suggest you see a doctor as there may be problem of some sort.

All the best!