Sorry that person was so irritating.
I was a part time receptionist at a 500 employee sized company, it can be so busy one minute and then so slow the time doesn't move at all..

But they always expect the front desk personnel to be professional and pleasant..and cool under stress..

Take some deep breaths and forget about what that person said, maybe they had a bad day themselves.

I think in the future I wouldn't mention your health situation though, someone might try to confuse you on purpose and take advantage of the situation.

Just cover a slight mistake with a simple "Oh just noticed a opening at such time" nobody needs to know why you didn't see it before