Well take this with a grain of salt cause I don't know jack about this but it sounds to me like she's feeling a bit slighted or edgy when she can't get you whenever she wants you, and is now playing games with you to make sure you're still on the hook.
Sounds to me like instead of offering explanations or justifications why you didn't call, you could just respond with "Sorry, didn't notice, was wrapped up in something else," etc. Shrug it off, blow it off, let her walk away and, like many here have said, let her stew in it a bit cause the only way to win this game is to refuse to play it.
She wants you to follow her out to her car and justify...I don't know if that's a good sign or not, but it sounds like she's feeling the loss a little to me. That's probably a good thing.
You keep using that word. I do not think it means what you think it means. -- Inigo Montoya, 'The Princess Bride'
