I can't say for sure what it means, since only Erin could know that, and there is a good chance Erin is pretty confused too. Lots of times people get overwhelmed and sometimes they react this way trying to get the situation to be manageable.
I would like to make this clear though, your fears and trepidation about Erin's potential transition are totally valid, I mean, it isn't every day that this sort of thing happens, and maybe it would work out great with you both being girls maybe it wouldn't with that level of ambiguity it would be a near miracle if you didn't have concerns.
Of Course her feelings are valid too, it is only natural to feel a bit hurt and betrayed when someone who you trust and (i assume) love isn't sure about if they can handle being with who you feel you really are inside.
Honestly it is a bit hard to see why they asked you just to be friends, it could be a (bad IMO) way to try to protect you, as they might have taken your uncertainty a bit harshly, and could feel a bit guilty about "putting you through more" or perhaps maybe they are trying to find an easy way out. Or perhaps she was just overwhelmed and couldn't deal with things and decided to pull back and isolate herself a bit to try and get a handle on things.
In any case, really no one can know the exact meaning of the action, I wouldn't focus so much on what Erin does since you can't control other peoples actions. If you want to still be together tell her that, and be totally honest about your fears, and understand that your fears about things going forward may make Erin feel less confident about her future with you, but totally honesty between the two of you is probably the most important thing right now.
I know this isn't much help, but just remember, you both have valid feelings, and sometimes things just won't work, but sometimes if you both put forth the effort, it does. I wish you both the best of luck, regardless of where it goes.