When my son and DiL had their baby, her mother had it in her head that she was going to be in the delivery room with them, even though it wasn't what either of them wanted, and she hadn't been invited to be there. I even overheard her telling her friends that she was going to be in there mopping her daughter's brow! My DiL is very sweet and kind-hearted, and finds it difficult to say things that might upset people, and my son just wants to keep the peace. As it turned out, they had GC overseas and the hospital would only allow fathers in the delivery suite, so she didn't get her own way, thankfully!
I don't see anywhere in your post that you had actually discussed with your daughter and SiL and been invited to the hospital ... had you simply assumed that that was what you would do, without being asked?
A couple having a baby is one of the most special, intimate and emotional times that they will ever experience together. It is for them to share, just the two of them, unless they BOTH decide otherwise. They need to discuss it between themselves, and if they want someone else there they will ask. If you are not asked, then please don't assume that you can go along and be part of what is their special time. I personally don't think you should ask your daughter outright why she was shaking her head, she still may not want to upset you if she doesn't want you there, and she simply felt her husband was being a bit blunt. Much better to simply tell them both that you're sorry if either of them feel that you've overstepped the mark, due to being an excited GM-to-be, and that you're happy to follow their wishes and visit your new grandchild when they are ready to call you. Exciting times ahead!