I am afraid of the dark...?

I am very scared of the dark... no it's not the boogie man... I feel unwanted things around me... like alot of things, or some type of object is hovering over me. At night, if the power goes out, I start screaming, and crying... I honestly don't know why... but befor that happens, my heart pounds so fast, I get lightheaded... It's so wierd how I react! Can anyone tell me why this happens? D: