How did I test positive in a lab test for meth, but I didn’t use.?

I have a roommate who shares a bathroom with me and uses, but I don’t know how I could have gotten it from her. I also had sex with someone who had probably used, and he finished in me. Can I really test positive from something like that?

I take Wellbutrin which can show up as positive in a drug test, but if it was confirmed by a lab - doesn’t it mean it couldn’t have been my medicine?

I use no other drugs. I am in a court program and this has happened before. If I had done it, I would have confessed and received a lesser punishment, than I am for telling the truth about not using.

Please help me to understand this!! I do not want this to keep happening, so I want to address whatever may be causing it.