Failed a drug test for Randstad what happens next?

I haven't used MJ in four months and took a urine test, I got a call today saying I tested positive for it despite doing absolutely nothing. I don't understand how I failed in the first place but I called the company and explained what happened and they were unable to give me the job. I'm scared as to what's going to happen now. Am I blacklisted now and won't be able to get another job? I quit four months ago specifically so I wouldn't have to worry about job hunting.