help! failed pre-eployment drug test.

I had a pre-employment urine drug test on wed. and failed. I took a tylenol #3 the day before, not even thinking. The problem was the prescription is not mine but my husbands. It was an old script that I found in the cupboard , motrin hadn't worked for my headache so I took one.( I know as a nurse I should know better ) I have never taken any drugs...illegal or controlled sub. of any kind.I never have a second thought about the test because I've never taken any drugs, ask me to pee in a cup and I will, i had nothing to hide. It was a very stupid mistake.I now have to wait all weekend to talk to HR, cause the employee health people said it will be their decision if the decide to retest or recind my offer.
I am a new grad and had finally landed the job now I'm afraid that I'll lose it.
Anyone know if this may be a forgiveable offense? I have never failed a drug test and honestly never taken any pain meds except after childbirth.
PLEASE do not respond and tell me how stupid I was or how I should have known better. I am sick about this and am only looking for positive advice or responses.