Unreadable Drug Test?

I went in for a drug test a couple weeks ago for a job and was told today that it came back as unreadable, and I may take the test again. I don't understand what this means? I've never even once taken drugs nor any medication which has not been prescribed. It was a urine test, but I've never had one. My past job simply used my hair for a drug test, which I would think would be better anyways since it's more extensive. I do take birth control, allergy meds, however I was on my menstrual cycle at the time, would this have affected anything? They didn't ask me for a list of medicines I take, either OTC or prescribed. Should I tell them that I do take medication? Any help is much appreciated.