Why do I weigh so much but look so thin?

I'm 13 and I'm embarrassed to go back to the doc when school starts again... I'm not fat I look very skinny it really makes no sense why do you think this happens I am pretty tall for my age 5'5 but I'm not sure if my scale in my house is bad and I always forget to weigh myself before I eat so I'm not sure how much I weigh prob between 120-140 but I'm not fat all any ideas?