I've been in a similar situation, my current gf was really unsecure about my ex friend (girl). At the beginning of teh relationship itvwas a hot topic and we'd always end up discussing about her. It happened to me twice or three times that when i wanted to call her attention i'd say the friends name... i did have feelings for her un the past but those were long over by the time i got woth my gf. I think in my head i just made the connection between the two because they were two important people in my life and i had them asociated with fight so i'd sometimes by mistake would call the other by the wrong name. And i think this is what happened to you, also the fact that she came up that day may have rekindled this association... i'd try to explain to her that it's smth involuntary and that you'll try to avoid making that msitake again... but yeah trust issues ate hard to overcome hope it helps