i havent read all the posts but heres my advice
i had a bit of a bad first experience, in a vanilla relationship. about 1 month into a relationship i woke up from a nightmare/flashback and started hitting my b/f telling him to get the hell away from me. you really have to check there was nothing bad about her, anything she hasnt said, it could even be as simple as her mum or dad smacking her when she was a child, explore all routes possible. i think its really important not to let it happen to many times, if it does happen again be there for her, take it as an ideal opportunity to talk to her (i dont believe its not for want of trying) but you take the lead, realising you are once again vulnerable is a scary thing, maybe thinking someone is displeased with you could be the trigger. i have a lot of friends who have been abused or have had even the slightest of problems who find vanilla relationships hard so it comes as a given that a BDSM one would be hard (it was an online community btw)

hope it helps
emma