I know exactly what you mean, my very difficult daughter still blames us for anything that goes wrong or difficult behaviours from her two for the following 24 hours after we've had them, and they are 3 years old now! We've looked after them since they were a year old and their mum went back to work. We do a lot less now they're at nursery school, and in fact I miss them, as I don't see so much of them unless we ask to see them. DH sees them at least twice a week as he goes up early to walk their dog and usually sees the children then. I don't particularly want to get up at 6.30 to go and walk the dog so I don't see them.
I think Dukey that as others have said, she knows that you are the best possible person to look after her baby, and she probably feels guilty about leaving him, and probably jealous that you have him when she really wants to be with him.
I have found that it's best to try and let the criticism go in at one ear and out at the other, whilst trying to follow her rules and routines at the same time. It's complicated I know, but I've been there and I know how painful it can be when as a loving and caring grandparent, you are criticised for doing your best.