If I haven't used any methadone, drugs or meds why did my instant 12 panel test show positive for methadone

Don't know; you would be the best judge of that yourself. It isn't like alcohol where alcohol is used as a stabilizer in many cough medicines.

Those tests are meant to be a coarse screen. They usually follow up with a specific test that is sensitive and isn't subject to some of the holes of the initial screening. The panel says there's methadone, and the second test should say how much, or show that it was a false positive. Are you aware of your legal recourse if this is in fact a false positive? I would learn about that in case you're caught up in some legal repercussions of the test. If it is a valid positive, then accept your punishments.