I cant sleep! Why is this? Please help!?

I am a 13 year old girl and every night when i go to bed i cant sleep i will lay their for hours before i finally fall asleep my back hurts and sometimes my neck hurts this has been going on for almost a month and im starting to get mad i dont know whats wrong but i have been unser alot of stres and im scared that i may get held back and that im fat cause a girl keeps calig me fat but when i asked my mom she said im not im just scared that i am but why cant i sleep?