gransal, you must be hurt at that. Try to keep your own counsel, it's really hard, but it will save a lot of grief later on.
What you don't know is if MIL turned up anyway, perhaps DIL was tired and her mum stepped in to help and she was too exhausted to say "No". When it's your own mum, you don't need to put on "a face".
I was going to say that a new mum usually wants her mum around her for support, but in my case it was the opposite. I'm the MIL and my DIL wanted me and not her own mother to help, which was quite tricky.
It's still the same three years later,
Try not to let it upset you, ask what you can do to help: shopping, washing and ironing, letting mum go for a sleep or shower.
We get caught up in the euphoria of the birth and then reality sets in.
Get some sleep and you will be better place to support your son and DIL. DIL's hormones will be all over the place, she is probably feeling a bit overwhelmed at the moment.
I think new mums are sent home too soon, nowadays.
Remember, tomorrow is another day!
PS, Erin is a delightful name!