I am so sorry to hear about your loss, especially when it sounds so unexpected. It leaves so many questions left unanswered and us wallowing in our grief. It makes it so hard :(
You can't beat yourself up about not being there, you had no idea.. and even if you did it's okay to not be by her side. I know that sounds weird, but you have to be able to forgive yourself. She wouldn't want you to be mad at yourself. Again I'm deeply sorry for your loss, and we're all to here to talk if you ever need to!