I feel like I have cheated. But I am not going out with the girl?

since you werent actually together technically you havent cheated. obviously you feel something for girl A more than just friends and im sure youve thought about taking the relationship a step further or have already planned to in the near future. this being said your mind set is that your already together, youve gotten along great and have gotten closer to one another. therefore by doing what you did with girl B you feel you have cheated. Your not in the wrong it just feels that way. but its good that your going to tell girl A about what happened, if your going to start a relationship with her its right that you let her know what happened