It is never easy to be put on the spot...especially when you are asked what mistakes you have made in the past!
I think you did fine. As long as you didn't say something along the lines of, "You know, I was really nervous at the end of my shift because I had a date with this guy that I had been DYING to go out with, and my brain just wasn't working right. I like, TOTALLY forgot to give my 1800 meds. But it was cool and stuff because I DID report it off to the oncoming nurse, and she completely understood. She told me that she would give all of the meds as soon as she was done taking report."
Med errors are certainly nothing to take lightly. The point of my silly quote is that you DIDN'T do this, but you admitted to an error (accountability) and then you took the necessary steps to correct it (responsibility). You further developed a plan of action to ensure the error was not repeated (prevention and safety measures implemented). Shows good judgment on your part.
I think your answer was perfect. (Bravery)