Thanks for responses. I'll try and answer the various questions :)
I don't think it was a surprise, as she knew where she was going, but it is possible that on of the Bridemaids or MoH was in charge of the guestlist. But It wouldn't have been a surprise to her as I wasn't invited, because I asked quite a while before the event if she was having one, so obviously knew nothing about it.
The other hens were a mix of the bridal party and other women- it seemed quite a mixed group. I have met most of them before.
I am pretty certain she would have wanted a hen party, rather than being reluctant about having on- she hasn't explicitly said that to me, but she posted a few things about being excited for it on Facebook, and I guess she is the sort of person who would want to do a hen do (sorry that's a poor way of wording it, it's not meant to sound negative- I just mean from knowing her, I would be very surprised if she wouldn't want one)
I'd don't think I haven't behaved in a way that would mean she wouldn't invite me, but I'll reflect on it- I agree asking her opens me up to the possibility of hearing something difficult to hear.
She was my MoH three years ago, and she was really enthusiastic/touched about it. I hadn't thought our friendship has changed since then- we have both had quite busy periods in our lives since then (moving cities, travelling, buying a house, job changes), but have made an effort to be in touch and meet up fairly often.
I think I might follow @ConkerGame's suggestion. I hate confrontation and tend to avoid it where I can, but I think asking gently might be a way to stop me wondering for years later what the hell happened!