My mom and dad always call me fat.?

For a little while my parents have been calling me fat. I don't think I am but they always say it. I'm 15 years old, 5'9 and 120 pounds. I play softball, soccer and snowboard so it's not like I don't stay active. I always go for a run after dinner to so I don't know why they are saying this. I don't even eat a lot, actually I'm not a big eater at all. Sorry this is so long but I'm sick and tired of them being jerks. What should I do? (I've tried to tell them to stop but they just laugh)