The concept: A girl documenting herself because she's experiencing memory loss, strange markings, etc. Throughout the story, there are times when the camera goes static and skips to later in the day, unbeknownst to her. As the story progresses, however, these skips are replaces by video from the earlier episodes: It's of her, but she doesn't seem like herself. There is something inhabiting her. She is naught but a vessel for The Higher Power, and she doesn't know.
Other idea:
A girl who frequently explores decides to start recording her nature hikes because she's noticing strange things. In the backgrounds of the videos, there's a blur of movement sometimes, but nothing big at first. The father the series gets, however, the stranger things are, until she vanishes for a week before returning with a video. She's seen the creature in the videos, face to face, and... it's friendly. It looks terrifying (think Daisy Brown's Alan, but the size of trees when on all five limbs, it's body thin, disfigured, and looking like a tree) but it's herbivoric and peaceful. She bonds with the creature, lovingly nicknaming it, but soon discovers she must protect it from another strange creature in the woods.