When your boyfriend calls you his ex's name?

We have been together 6 months. As far as i understand he hasn't seen his ex in 2 years and she married one of his friends. He is going home for a visit in a week. Last night while snuggling i sighed and he was like what's wrong sarah? he stopped and i stopped for a moment and he apologized several times. i said ok and sat there awhile trying to figure what it meant out. Do i remind him of her...he told me he loved me and held me tight. I know we all make slips. I'm not mad he's human. But it does make me wonder if its a red flag of where we are going. what do you think?