My doctor can't or won't prescribe the only medicine that helps me?

A little over a year ago I went to my doctor with a rotator cuff injury and she was going to prescribe vicodin for the pain, but before she could I had to sign a contract and do a urine test. Like an idiot, I did the urine test and it came up positive for pot which I had done 2 weeks earlier. Now I am having back problems,pain radiating down my legs,etc. She has prescribed me tramadol and ibuprofen but sometimes the pain is bad enough that those medicines don't work. There have been a couple times when I had to go to urgent care because it hurt so much. What am I supposed to do, I don't think it would be a good idea to tell the urgent care doc the reason my doc won't give me anything stronger for the bad days, but I am sure they are wondering why my doctor is not helping me. What should I do? I don't want the urgent care to think that I am going behind my doctors back to get pain medicine, but she can't or won't help me. What is the deal with that? Can't she give me another drug test? Should I tell the urgent care doctor my dilemma? Please help. Thanks