My mom is getting worse and it is hard taking care of her now. I don't know what to do.

I come over everyday and care for my mother who has dementia and real bad short term memory. I do have caregivers that relieve me during the afternoon and graveyard. But this particular past week has been real hard because everyone has been getting sick and I have been covering their shifts. I have been spending over 12 hr days with her and yesterday it came to a head. She has been verbally abusive to me for no reason in the past but last night at about 7 pm I was locking the patio door and I turned around and she appeared behind me saying that I was going to be very sorry and that she was going to tell my older brother about me. I said nothing. She went to bed and when she gets this way she really scares me so I kenneled her dog and shut off TV and sat at kitchen table in dark til graveyard person arrived at 8 pm. I was too scared to make any sound that would set her off again. I'm scared and alone and don't know what to do.