Congratulations on becoming a Gran. You don’t mention if the baby’s father is on the scene. If he is, could you have a quiet chat with him, and ask if she is like that all the time, or just with you? I remember feeling very isolated when I had my first child. A husband working three shifts, so I had to try to keep the baby quiet while he slept. Also in a new area, with no backup from friends or family, and the coldest winter we had had for many years, in a poorly-heated house.
Looking back, I’m sure that I had post-natal depression. I knew nothing about babies, and I think that my nervousness probably made the baby more difficult.
If you think that she isn’t coping, I would try to contact her local care team, discreetly, and ask them to call.
I hope that things improve soon.