Your grieving does not trump her right to be elated at her pregnancy. She can choose to announce it however she wishes. Perhaps she thought you'd genuinely be as elated, as her.
I don't understand this rather common theme on mumsnet that everyone else must mute or otherwise downplay their good news, for other people, regardless of their (terrible) circumstances.
Her becoming pregnant does not take away from your grief in any way, shape or form. It's a terrible loss and very hard. I'm afraid to say though (and I'm sure I'll be absolutely crucified for this), the world does not (nor should it) just stop because an individual is grieving.
I don't believe in muting her experience in any way, shape, or form. Not only will it not bring you any comfort to downplay her pregnancy, it's also not fair to her either. You never know, this may be the only pregnancy she ever has, and she deserves to enjoy it, and moreover appreciate it, to the fullest, as every mother does. I'm so sorry this was taken from you. Taking part of the experience from her in in the sense of controlling how she behaves happily around her pregnancy, won't bring it back for you love. It won't bring you anything except an elephant in the room and quite possibly a damaged friendship.
The PP who attended her triplets funeral and had her cousin phone her to tell her the news the same day - that was insensitive and could have waited a bit. If she was talking to you ALL the time about this pregnancy, despite having a word with her about it, then that'd be insensitive.
This is a hard one because it will be extremely difficult for you during the grieving process, but actually, I also think she really deserves a friend to be elated for her too, and to not feel she must tread on eggshells excessively about this.
If you don't like how she announced her pregnancy to you, or you don't feel you can be a proper friend to her during this time, then I would think stepping back would be the best thing for both of you, to be honest.
I'm very sorry for your loss.