Why HR takes too long to call me back?

So,I got a job offered at this particular SNF a couple of weeks ago and the HR did a drug test on me.She said she would give me a call back as soon as she got the results.Its been two weeks already but i didn't hear anything back from her.As i have never used drugs in my life,i m so sure that my result won't make any problems.Could that be they found someone else who is more qualified than me in terms of experience and having IV cert.Does anybody have a clue what that could be.I m so frustrated and feel discouraged at the same time.