I can’t vouch for why you are yelling at this moment, but I know how you feel. You are trying to be heard. She can't hear. If you are a daughter, it is a scientific fact that raising your high-pitched voice won’t get you heard if your mother is old enough to have sensory-neural hearing loss in the high frequency range. If you are a son, well she might be more likely to tune you in.
I have no idea why, but in retrospect I anticipated that the point at which Mary could no longer be put in the shower would be the most important milestone for me in her decline. I had no problem with the concept or actuality of changing diapers, but I couldn't imagine getting her clean enough with a plastic tub of water and wash cloth.