Hey Dani, your grandma is old and is "losing her mind". Don't take it personally. She's just going through a tough time. (((hug)))) I know you must have felt awful, and no, you are not a terrible person. Maybe she feels ignored and unloved at this sensitive time, and she's not thinking straight and maybe if you made her feel special for a day or so, she'll feel better.
I know she's being selfish, but she's old and she'll probably not be with you very long. You'll likely have your child for the rest of your life, but grandma? Grandma needs to feel like you care for her. You are not a terrible person. Don't worry about that.
I feel awful for you that this happened to you. I feel bad for the grandma, too. It must be a really lonely time for her. :(