If you do recall what happened, and you do have an opportunity to respond to someone else who was witness, imo it's perfectly ok to say," You know? I'm not sure that I didn't... offend you or.... I really wasn't feeling well that day, things were affecting me adversely.... " There isn't a need to apologize openly. Odds are, they aren't looking for an apology anyway. I also don't think that they need a full explanation of PTSD. . . It might be best to chalk it up to something that happened. imo your mere remembering the event is progress in itself.