Looks vs. Weight in lbs?

I am 18 years old, and I weight 120 lbs. I am 5'5. When I tell my friends my weight, they all say that seems dangerously low for an 18 year old, even for someone short like myself. But when I look at my body I can still see belly fat. Much more than my friends have, who are ironically heavier than me. I am confused, because I know I do not have as much muscle as they do, but I am not scrawny. Why do I weigh so little, but look a chubby still. Is it safe to try to lose weight? I have an admittedly small frame, but the number still seems so low. Yet I am unhappy with how I look.