Why would my period stop suddenly? (PLEASE read details before answering)

I am 31 years old, fair health, no diseases. I have been menstruating since I was 13 (regularly, like clockwork) and have relatively trouble-free, pain-free periods. I had a child at 17, but no children, pregnancies, miscarriages, or abortions since. I have always had good pap smears. I haven’t even been very sexually active for the past year (that’s a topic for another thread, so please don’t ask about that).

About six months ago, I stopped having my periods altogether. I have taken two pregnancy tests within six weeks after noticing this, all coming out negative (besides, the last occurence of sex was too far away from the time it was apparent I was ovulating).

I am not asking for a diagnosis; just some brainstorming, and maybe some theories from some of the kind Jelly-doctors within the Fluth-ocean.

So my questions are, What would cause this to happen in a 31-year-old woman? Has this happened to any of you ladies?

Let me add that I was forced to drop my insurance last year because the county stopped matching the premium…I simply could not afford it.