Out of 12 test I test positive 3 times in the past 2 years for Amphetamine.

Here is the low down

1. Never did drugs, and not even a single pill in the past 2 years.
2. I only test positive when I am on my period.
3. Took blood test right after words that came back clean, and a second UA.

Problem is I have to prove that I am not on drugs, but the second test don't count since I have to take it at a chosen place.

I am trying find out a real explanation on why this could be happening?