New Jersey - At least 1 of the 2 drug charges will stick to my record forever?

[s0beit]

So i had a conditional discharge for 2 counts of Possession of Marijuana. I was put on a 6 month supervisory program by the probation office. Shortly thereafter, i signed an admission of guilt stating that i had smoked marijuana a week prior to visiting my probation officer. I came back 30 days later and took a swab test. Ive been told i failed said test and dont know how. I have been clean during this entire time (since the day i admitted to smoking marijuana) and just dont understand. I now have a new court date for july and have some questions for the experts...

Mainly, how does this happen?
Is the swab test a reliable test?
Is it accurate?
Can the timing be too close together from my admission of guilt to the time of the test?
Can it be from anything i ate?

By going back to court, they will put me on probation and my conditional discharge will be over. Which means atleast 1 of the 2 drug charges will stick to my record forever. I am trying to join the Marines so that would be a very bad thing since you can not join the military with any kind of drug charge. That being said, what can i do to help my situation?

[forum_admin]

Double post.

New Jersey - Failed a Swab Test