Your girlfriend’s claim that she made an ‘honest mistake’ indicates you have discussed her out-of-place use of a formers lover’s name. The communication has opened. You are obviously concerned about the honesty of her mistake. Relationships are built on trust, not paranoia. If you want a serious relationship with this lady you need to be convinced her mistake was innocent and not her way of telling you she desires her former lover. Yes, you must continue the talk or making love with her will become a wait-and-listen game.
Do not consider this talk a negative confrontation. You have an opportunity to open to each other; use this communication as a window into her, and your, fantasy world. Keep in mind that emotions tossed out during the intensity of orgasm are seldom connected to reality. Her former lover may have introduced her to the wonder of sharing orgasm in the comfort of a man’s arms and her use of his name during this intensity could very well be an expression of the joy you helped her reach and not a longing for this former lover.
For most lovers vocal expressions during orgasm are undefined moans, groans and tones without meaning. Forming words is more theatre than reality. Using the lover’s name may be her pathway to orgasm, the final push-to-the-peak so to speak that became a habit with him. In this case his name is her conditioned response to reach orgasm. It is not him she wants. It is the feeling she learned with him that she seeks to recreate.
Solving problems, honestly, through uninhibited communication is how couples bond and grow. As you court each other, she will have many ways to show that it is you, not him, she loves.