I'm surprised that so many people seem unable to imagine a mother/grandmother behaving in this way or are alluding to the OP having brought it on herself in some way.
Sometimes, in life, and despite our plans/expectations to the contrary, shit happens. And, when it does, many people rely on family to support them through a brief 'hiccup'. that's how life and families work.
My mother did similar to me when my baby was 5 days old. It wasn't ideal that I ended up back 'home' at 36 weeks pregnant when my fiance kicked me out and I realised that having me there at 24 wasn't going to have been in her life plan. As such, I did my best to minimise the impact on me being there - I did the housework while she was at work; cooked dinner for her when she came home; had no expectation of her 'helping' me with the baby once he was born - and made that very clear. I required no support from her aside from a roof over my head.
But she was making plans behind my back to make me homeless and did so. I only discovered it when my son and I were discharged from hospital when he was 3 days old.
People don't talk about this stuff in real life, which is why people only hear about it on here. If you knew me, you'd have no idea that this was my experience.
But she wasn't a good mother or a good grandmother - she made attempts to sabotage me for a number of years after I moved out, whilst I made futile efforts to build/repair a relationship with her.
Eventually, her behaviours brought her/us/the children to the attention of SS and, following an Initial Assessment, the SW and I agreed that the children would have no further contact with her for their own protection.
Some people just aren't very nice.