Her hearing your private thoughts was an accident on your part. But it was not an accident that she listened. She should have hung up as soon as she realized she was listening in on your private thoughts.
You have nothing to explain, like I said these were private. You were alone in your car, so basically you were alone with your thoughts and your thoughts should be free for your speaking. If she questions you about it, don't explain yourself. Appologise that you made in error in hitting the dial button and nothing more.
If she is genuinly interested in what you had to say and wanted to speak to you woman to woman, that is your call.
Otherwise, no explaination or appology is nessesary here.
Hope it all works out for you
Your friend,
Prettylady