As far as your periods go- definitely stress can do this, and also you coming off the pill. When I was a student I missed a month altogether doing papers and exams, and I was definitely not pregnant, and you're saying that you're so obsessed with whether you have candida, you insisted on getting the tests twice? Sounds like you're wound up awfully tight about this. If the tests say you don't have candida, then maybe you really don't have it!
But it definitely sounds like you might want to do some cleansing and push out the bad substances out of your body and from your emotions.
What seems to be the going wisdom around here is that you cleanse your bowel (see the Bowel Cleanse Forum for ideas- PB Shakes are the cheapest, and Bentonite kills yeast too) while on the candida diet and rotate different candida killing substances- like garlic, oil of oregano, caprylic acid and so on in order to get most of the strains you may have.
Plead read the Best of Shelly- she has a lot of wisdom on diet and how to take care of your bowels and Candida.