Keep seeing silhouettes of people out of the corner of my eye?

For the past few months, now. I have been seeing shadows of people out of the corner of my eyes, whenever I turn to look at them, however, they disappear, also, in my dreams there is always one constant factor, a shadow that follows me. However, it has gotten worse, the other day I stood in the rain staring at a shadowy figure that just stared back. I've been hearing very fast whispering too, what could the problem be?