What do you think of a girl who's 5'7 120 pounds?

I'm fairly toned all around my body. I'm 17 years old and I fit sizes 0-2. I heard some girls are a size 6 at my height and weight. When I started dating my ex bf I was 125 pounds but since I got to 120 pounds my boyfriend saw my ribs when I laid down and thought it was gross and that I needed to gain...I know size 0 is suppose to be really skinny, but when I look at myself in the mirror I swear I'm a size 4!