Serious sex question please help me!!!?

Ok, I'm a 22 year old guy and every time I have sex with my girlfriend I let out this blood curdling scream. I'm not in pain or anything and I'm not even really sure why I do it but it happens every time. Never fails. And when I say "blood curdling" I mean BLOOD CURDLING. We've even had the police show up because the neighbors thought something was wrong. I wish I could describe it better. But I can't. So yeah, any suggestions as to why I do this or how I can stop it? thanks.