My daughter gets them every so often, she will start screaming (real piercing ones) then I will rush it to her room to see whats wrong, she will be staring at the wall or the curtains then freaking out and trying to escape from the monster/ghost/animal
I have found that she seems to get these when she is stressed (she had tests all week and she broke friends with her classmate)
To a lesser degree I get a similar thing where I keep waking up and sitting up bolt upright in bed and I see all sorts of strange things (shadows,distorted furniture), I have pinpointed this down to stress as well.
I sympathise with you though it is awful!!, I cant offer much in the way of advice except to try to relax more, de stress your life, dont go to bed over thinking about stuff (my daughter does that)
My doctor advised me to take a sleeping pill (like a herbal one) to relax me more if it got too bad, although I am trying the de-stress route first!