A game of chicken, that's it exactly. As a rule of thumb if you take it that she's right you're wrong, that's probably the sum of it.
You: Oh hi mum! (Daringly) I was wondering why you haven't been in touch? Is your phone working okay now?
Her: Why's it up to me? I've got my own life you know, etc etc. I can't be expected to run after you all the time, etc. It's all right for you. you've got DH and the DCs, I have to fend for myself, etc.
Or
You: Hello Mum, how's it going?
Her: (Weak voice) Well I'm glad you finally asked, I've had XYZ go wrong and it's been terrible, I don't know how I managed all on my own.
You: Oh dear why didn't you say?
Her: (Forgets weak voice, extreme irritation) Well I shouldn't have to ask should I, etc etc. I could have been lying there unconscious all week and no-one would have known, etc etc.
I may have her completely wrong. It's a shame but to some extent it's like teaching DCs consequences: when we play nicely, things go all right and playmates remember and are glad to include you and ask you to join in again.