(1) What about, "Thanks, I feel very unsettled when I'm afraid you are talking to OM, so your openness really helps me alot. Thanks for the support and understanding."
(2) You KNOW do you????? Did it ever occur to you that MAYBE she talks in her car for privacy and MAYBE she needed to have a private conversation with her sister because she is going through alot right now or MAYBE she went to the car to call OM but resisted and called her sister instead and was feeling very good about that until YOU took her down a few notches.
Oops, I'm sorry, you said you KNEW she was talking to OM so I'm sure you have absolutely convincing evidence that those, or any other, possibilities are false.
And, I'm certainly glad you are being silent rather than addressing this simmering crock of sh*t directly at this point. Nothing breeds intimacy, love and trust like resentment. How long do you think it will be before she stops trying at all when you shoot down her smallest efforts? Why would she be willing to take any greater risks?
I suggest you go apologize to her immediately for your passive/aggressive and sarcastic sh*t. That you admit it was a zinger that you threw at her because of your own hurt.
You know, she already hurt or she wouldn't have had an A. If you are going to punish her for the A by zinging her everytime she tries to be more open with you, you may as well call it quits right now.