It happened to me when I was almost in your age. I was about to fall asleep and suddenly I thought I was about to fall from somewhere or fall into a hole and stuff like that and I jumped alittle and sometimes almost sat in my bed. But it doesn't happen to me anymore. Right now other things wake me up.
Anyway I think your problem will sort out as you get older.And Perhaps you'll get into other kind of problems problems like insomnia or not having enough time to sleep!