How do I stop unwanted thoughts?

Before I Start have not been diagnosed with OCD or any other disorders ('cause I haven't seen anyone for it). So I've been having these horrible thoughts. Mostly at night but sometimes its in the day time. The thoughts consist of me grabbing a knife and stabbing someone in my family, or grabbing a belt and strangling someone (or other things but I won't say.). It;s effecting my sleep because i'm afraid if I go to sleep after having these thoughts in my head I'll do it while I sleep walk (I sleep walk a lot.). Yesterday I even had it while my sister was talking to me. There was a belt behind her and I had a sudden thought of grabbing it and using it on her. (My hand even twitched.) I'm so so so scared. I can't take this anymore. I tried telling my mom but she doesn't believe me and doesn't understand how horrible this is. I'm not a violent person, I love my family and would NEVER EVER do or think something like that. Help me. What do I do??