Emma,
I'm so sorry you had to go through that! It's horrible to be confronted like that in any situation, but especially in a group therapy session that should be supportive. I hope Susan can do something to resolve it in a fully satisfactory way.
Personally, I can certainly understand the desire to understand why Pamela did what she did, but it might just lead to greater frustration since you can never be sure unless she tells you. If she's already prone to be like that it might have been made worse by something going on in her life. I don't mean to suggest that as an excuse, it's not, but a possible explanation.
I think the most important thing to remember is that whatever it was, it was on her and you shouldn't internalize it (and believe me, I know that's easier said than done, i'be let myself feel hurt over far less incidemts).
At least you have this space as an alternate venue, certainly not a substitute for in person meetings, but it's somewhere to turn.
XOXO
Christie