If your bodyfat is too low, your hypothalamus will receive too weak of a leptin signal from your fat cells and shut down GnRH secretion, which activates the rest of the hormonal cascade (FSH, LH) leading to estradiol production in the ovaries. 5'7" 110 is extremely lean for a female.
Additionally, intense exercise increases zinc depletion at a greater rate (which would have the same result as too little bodyfat), and if you have endured periods of low zinc intake due to dietary restriction in general, but especially as a result of a veg*n diet in the past, you could be pushing yourself into a significant deficiency. Simply eating meat would either take a very long time to correct this or possibly not work at all. Because of how crucial zinc is for the development of a the fetus, you probably wouldn't want to get pregnant if you were deficient anyway.
The correction of a deficiency-related amenorrhea via zinc supplementation can take a long time as well: http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0447.1990.tb11077.x/abstract
In 13 subjects the menstruation returned 1–17 months after the initiation of zinc therapy.
You wouldn't be as deficient as them of course, so if this were the cause, it shouldn't take that long. It'd be better to overshoot though.
There are obviously a bunch of other issues that could interfere, but these are easy ones to address first.
