Hi there ~
I understand your concerns. Using tobacco and weed at the same time is a common "high" that many people like to experience. However, it is not safe to do so even in a single shot sort of setting. As you yourself have experienced, it leads to paranoia
and you may end up losing your job over a small amount of substance that was detected in your urine. I believe you need to be more careful and kick the habit before you lose out with your job and life. I hope you consult a psychiatrist
as soon as you can.
I hope this helps. Take care and have a lovely day!