Congratulations on the new gb (grandbaby)! Is this your first?
As a pp (previous poster) said, it seems early for dd to be back at work. If she isn't, then she may not feel any need for a babysitter. If she is but isn't taking you up on your offer, perhaps she already has someone minding her child. It's even possible that she and the dad trade off childcare and work hours.
Has she ever said anything to you, at all, about babysitting? Has she ever indicated that she doesn't need/want any babysitting right now? It seems odd and a little rude that she wouldn't respond, anyway, even just to say, "No thank you." But if she has already made her feelings known on the subject, that might be why she's not answering, Idk.
I'm so sorry you are feeling "left out." But for now, I would take her silence as meaning that your help isn't needed/wanted at the moment. As pps have said, there is plenty of time. Please just wait for her to reach out to you.