An awful situation, Nailit and in such circumstances your anger (together with what I imagine are a myriad of other feelings) are understandable.
If we are at end of life, it is sadly sometimes very unpleasant for those close.
Try to work through the anger and remain calm for your mother. You are probably knackered and existing on adrenaline so that isnt easy. If you can, get hold of her GP since she should not be sobbing in pain (although, in fairness, it may not be pain, it may be delirium from the morphine which has really odd effects on people - I am no doctor, I say this from personal experience).
Keep talking to mum. She may be more comforted that she can show by this and you may in time seek some comfort from the fact that you said whatever you needed to to her.
It's a *** situation. I wish her painlessness and you strength.