Why do I scream in my sleep?

I'm 18 now, but since I was about 10 I've been screaming in my sleep nearly every night! And it's got worse as I've got older. I don't know what's wrong with me because I don't remember having a nightmare of any kind. I bought the iphone app sleep recorder and found myself screaming every night something along the lines of 'no no please don't hurt me!' Is there anything I can do to try and calm myself before I go to sleep? I always have a vague recollection of screaming when I wake up, so it's not like I don't know I'm doing it! Any help would be greatly appreciated :)