Sounds a helluva lot like you've got eye floaters. They're mostly nothing to concern yourself with, unless you've had sudden changes. See - http://www.webmd.com/eye-health/benign-eye-floaters
The black circle in the shower is very likely to be one, particularly if you've got a bright white or light-colored shower curtain. Essentially what happens is, you look at something brighter, and the floater looks darker and you get more contrast. It seems more obvious to your vision.
As for the remainder, you've got an active imagination, nothing more. It's happening, it seems, when you are alone and it's dark out. Our minds often play tricks on us, particularly if we're a bit fearful. And that is highly likely to be all it is.