new job drug test!!

I just got my first real job offer ata hospital! I had to go in this afternoon for a drug test, I don't drink/do drugs so I wasn't worried. I drank water so that I could be in and out but I somehow froze and couldn't pee! I sat in the room for 2 hours and peed too little they said. But they said if I can't go anymore it may be a positive since I tried avoiding it (I couldn't pee! I drank water had a cup of coffee!) what do I do if I have to go again? I'm so scared because I know I'm fine and can pass they just made me scared! I asked if they could do a blood test and they kept saying no. Can I get a doctors note for future??