I know surgery and stress can do strange things to your cycle. The last time I had surgery (it wasn't planned) I had just started my period. They told me in the hospital not to be shocked if is stopped, which it did. Several weeks later when I went back into surgery to be sewn up, they told me not to be surprised if it started again. And it did.

And knothead is right about the vacation thing. As a teenager I worked as a maid at a resort and every woman there had Aunt Flow visiting. I remember thinking "why wouldn't you plan your beach front vacation around that time of the month?" But sure enough everytime we've gone on vacation Aunt Flow has to come with us, even if she'd been there the week before.

But with shingles it may be the other way around, you may be having an outbreak because your immune system is stressed dealing with the pre-cycle hormones.