I think I'm fubared

Well it's been exactly 4 weeks since I smoked and I tested positive with one of those thc dip cards. I don't see how this can be possible since marijuana supposibly stays in your system for 30 days. As I stated before I'm very thin and have a high metabolism so I don't see how this test can be correct. I take the real piss test tommorrow after work. Thinking I'll drink a couple Arizona ice teas and pop a couple B complex vitamins 2 hours before the test. I also have a 2 oz bottle of synthetic urine and gonna try as a backup plan. gonna use one of those pocket hand warmers to see if it will stay at body temperature.
Any thoughts or suggestions?