I would start with the "I'm sorry" part. If it was me, I might add, "I should have..." I may be overly sensitive, but "I was just" always hits a nerve with me. It sounds defensive (which is a huge trigger for me). Maybe it's just semantics, but I wonder if a phrase like, "I wanted to make sure S's birthday was organized, so I may have jumped the gun on planning without consulting you."
I think I would also avoid asking, "What can I do?" Didn't she sort of tell you. maybe either, "Next time I will..."
I'm sure vets will have great advice, but this was my initial reaction. (But I'm definitely no expert!)
