An overgrowth of yeast can be caused by antibiotics, stress over a prolonged period of time, lowering of immune system, for some people birth control pills, foods that are high in sugars for example breads, pastas. It may not have been one specific thing that caused your systemic yeast problem, but a combination of things. I really hope you start to feel better soon. Most times I read what your going through and I usually don't reply because I have no advice to give but I just wanted to say hang in there and be strong.