Shy bladder is a problem. Going to be either prostrate or your kidneys are not allowing fluids to process. among other things. You need to go to a proper doctor to run tests on that bladder system. Im presuming you do not do drugs or other things that dehydrate you. This is not something to fool with, this profession will treat you like a drug user these days. Regardless of the truth. I hope that for lack of a better word there is indeed a detectable medical problem inside your body so you can defend yourself from the retribution levied against you for refusual.
How can it be marked as a positive test when he didn't even supply enough urine to be tested? Perhaps a refusal but even then I wasn't aware that any trucking company can "disqualify" your CDL. Only the DMV can do that.
For a different industry.... I had to drug test for a new position. I only had 36 hours to get it done. I drank water until I thought I would be sick. The place had vending machines but I had to order pizza and a 2 liter for delivery because all my change was in my car. They wouldn't let me walk out of the door! Walking out of the door was an automatic fail. I sat for what seemed like forever.... But I finally went. Thankfully it was a 24 hour place.