Am I obligated to do anything or offer expiation if I spoke to a girl after having left her for the sake of Allah Almighty with the intention of not speaking to her again because she refused to marry me, and then, months later, I wished to propose to her a second time?

Your statement, "I left her for the sake of Allah and I will not speak to her," is neither an oath (yamin) nor a vow (nadhr). Rather, it is a promise that does not obligate you to anything. Thus, there is no harm in you speaking to the girl and proposing to her, and no expiation (kaffarah) is required of you if you uttered this phrase.