its never seemed like the right moment to mention it as she has been almost constantly wound up about something
I know it doesn't always seem this way, but you aren't responsible for her reaction. If she chooses to fly off the handle at something you've chosen to do, then that's her choice and her responsibility. You can care; you can be supportive and validate the way that she's feeling. But her being upset with you doesn't inherently mean you are doing anything wrong.
One of the common dynamics with individuals with BPD is that we often get pulled into justifying, arguing, defending, or explaining our actions or behaviors. This comes from a good place--we want them to not feel the intensely negative feelings they may be feeling, and we often feel as though if they only understood where we are coming from, they wouldn't be so upset. This is often unsuccessful, because you can't make them understand you. If you girlfriend wants to understand what's happened, I would explain it once; be wary of getting pulled into a circular, nonsensical argument where you are continually seeking to justify or explain your choice to do the quiz instead of football.
Good luck Yaffle
