I also think something underhand is going on. I've hear about 'Wendying' before (on mumsnet).
I don't think it is odd she should become friends with your friends, at all, or nasty or unkind or anything. What is unusual is that your friends, one you have known for a very long time, has started to avoid you.
In your shoes I'd want to find out what was happening there. I'd maybe make some opportunity to see the friend who is ignoring you, if you can, perhaps to return something of hers you have got or to drop round a cake or a present etc. Asker her what the problem is, in a really nice and neutral way, just say something like 'It seems we are...... lately... etc.
If you do this make sure you keep quiet, if you can! Don't chip in too much with 'is it this or that?' or 'is it related to J?' etc. Just be really neutral and lots of silence from you, with a big smile. I think people sometimes find it hard to resist a silence and seek to fill it.
If, in the long run, she has blanked you, and you do not know why, and cannot find why, I would move on and make new friends. I'd probably, in your shoes, make it clear that the friendship was important to me and I was always there if she needed me.
Maybe one day she will say what she felt, thought, what someone else said, and maybe for me it would be the desire to get at the truth that would be important, as well as not to lose a long term friend.
As to J, no idea what to do, part of me feels if you confront her she will deny it, but you could always ask! Maybe someone else who has been in those shoes could advise.
I hope that this is not the case but it seems it may be. Whatever happens, try and move on, don't dwell on it, if she is doing this for her evil pleasures your discomfort will be something she is looking for, and if not one is doing it, and friends are just moving on, them your deep upset will just upset you.
If you Google 'I've been Wendied'. you get a few threads similar to yours.