First death....

I work in a dementia unit at an assisted living. We had an 88 year old dementia patient, total care, likes to talk to herself a lot etc. the other morning we noticed she didn't look like she was feeling well, a bit pale and quieter. Staff had taken her bp and got 160/116 hr 76. From previous experience with this resident I know her bp can be off sometimes as she often shakes her arm a lot. I retook it myself within half hour or so of the staff taking it making sure to keep the arm still and got a reading of 117/70 hr of 50 ( her pulse often ran low). Sp02 was good at 97%. At breakfast she picked her food and drink up as she would usually do. Didn't eat as much just seemed tired. We laid her down until lunch. At lunch she still seemed a bit tired but had perked up and was starting to talk more like herself. She still didn't eat great but her color looked better and she seemed more awake. I had called her daughter that morning to let her know she wasn't feeling well and she was going to visit that afternoon. After lunch around 230 I get a call from staff that she was unconcious in her wheelchair and had died. I was in utter shock. Not even thirty mins prior she was talking to us and acting like her regular self... I've seen patients go through the stages of dying but never like this. I feel like there's something I should of done. As she started improving through the day I was less worried. I was going to double check her bp again right before they called me. I just feel terrible.