Help! False positive on urine drug test :(

Hi.. I need some advice in a bad way. I recently quit a job to go back to my old place of employment. I'd worked there for 3 yrs and they were so glad to get me back that they didn't even interview me. The job required a pre-employment drug screen. I do take ritalin for ADD and knew that if I provided a current script I'd be ok. Took the test, the lab usually asks what you're on but they didn't want to know.
Got a call from the 3rd party MD that the company uses and was told that I was positive for amphetamines (duh!). Per their request I faxed my current bottle label. Figured everything was all set and started orientation. On the 3rd day of orientation, HR pulled me out and I was told that on the GC/MS 2 other amphetamines showed up. I couldn't explain that and was terminated on the spot.
Called my prescriber right away (she was stumped too) and had her fax a requisition to the same lab for the same test. It showed ritalin only, w/ GC/MS confirmation. This test was taken only 3 days after the first, so other drugs wouldn't have had time to leave my system.
The BIG problem. They can't tell other prospective employers why I left, and I have a good cover story, but if I ever apply again in that health system they CAN tell them about the drug test. I'm a psych nurse, and this system owns about 2/3 of the psych hospitals in my state.
Does anybody know if there is a legal way to get my record expunged, or at least to include the test my CNS ordered? I had begged them to retest me and they wouldn't. I went on several promising interviews last week, and will have another job soon, but not being able to work for this health system will definitely have an impact on my career in the future.
I'd like to avoid getting a lawyer, but might have to, since they won't release the results of the test and I don't even know what the other amphetamines were that showed up.
Any advice or links would be very much appreciated!