What Causes Pin Pricking Sensation In Body During Mornings?

I wake up just before dawn with pin prickling sensation on my skin. Then where the biting feeling is there are droppings left behind. Usually black but sometimes orangish, little ball shaped hard droppings. I can t ever see what s biting me! It leaves no mark just very annoying and keeps me awake for an hour. Do you have any idea what this could be? I put a mattress zippered cover on, it worked for a few days only. Then back they were again. It Sting s at first and goes away showing no bites... Thank you....