Is it normal to have 2/3 rolls of fat when sitting and slouching?

I'm 5'7", thirteen years old and weigh 8.5 stone, or 119 pounds. When I sit and slouch I have 2-3 rolls of fat, and sometimes my stomach goes over by trousers/underwear by like 1-2cm. I wouldn't say I'm fat, but I'm not one of those skinny freaks whose stomach goes inwards.