I was in this dream and i was underwater and i was like in this war and on the other side there was this little boy who secretly killed people from my side and all the sudden i saw him in my head and i was almost about to wake up becuase something was telling me i was next and he was going to kill me and it was the scariest feeling ive ever had. I opened my eyes and i couldnt see anything but this huge black figure and it was comming to kill me…im really not kidding about this. I heard like the sounds of a large drum in a really low pitch but it was so loud. I tried screaming but nothing came out ( This is real life), and i tried moving but i was paralyzed i couldnt do anything but panic. I dont know what this was but i was too scared to close my eyes for the rest of the night. What happend to me?
Yes, this sounds like a form of the old hag. As you have discovered, even when you know what it is, it can be scary. If you are not 100% sure of what it is, or if you focus more on what is happening then what it is, or how you might use it to your advantage, you get scaried.
So be sure of yourself, and don`t get scaried. 
don