there have been times ive been in that position and i cant say why, but some things do act as triggers for me those are generally in relation to past experiences so we avoid them...is that a possibility with your sub?
it may just be that she was nervous before you started playing which does happen sometimes especially if its something or with someone new, then when you began the lid just blew (if that makes any sense?)
playing can lead to all kinds of different emotions, thoughts and feelings mostly i suspect because of the adrenelin,hormones and endorphins etc going haywire.
so if she's generally a person who keeps things in and doesnt tend to get too emotional its possible that shes a bit more vulnerable to it at those times.
its simply a matter of taking your time if things go wrong then as you did before stop,hold her and wait for her to calm down, people always say to talk talk talk in relation to playing which generally is a good thing but if thats something she's not comfortable with then maybe it's best not to push her and wait for her to talk to you.