Except that I used to have, or technically still have, premenstrual dysphoric disorder. I would always know when my period was due because a week to 10 days beforehand, my breasts would swell up into enormous, painful balloons; my belly would become a hard, tight drum; and my legs and back would start aching. I'd start feeling irritable, and this would get worse over the next few days until I'd go completely fucking batshit on day 23 out of 28. Psychotic thoughts, depersonalisation, and intense self-hatred. Eventually that mood would "burst", and from day 25 to 28 I'd be back to the irritable while crying uncontrollably at any minor provocation. And I would get ridiculously clumsy, to the point of non-deliberate yet dangerous self-injury.
Now I am on carbamazepine, a mood stabiliser. 500 mg. The mood changes barely happen at all - I suppose I may get a little more irritable or easily provoked or tearful, but it's nothing - a few minutes, maybe. My breasts do swell up, but not until 3 days before my period is due. I get clumsy, but only as much as I do when tired or "glandy". My back hurts and my legs feel sore, but no more than from fatigue or overuse. I don't know why the mood stabiliser should help reduce the physical symptoms as well as the emotional ones, but it seems to be a real effect. I could hypothesise that carbamazepine is also smoothing out hormone levels as well as neurotransmitters (it's known to stop many contraceptive pills working), but I don't know enough about the biochemistry to come up with a plausible mechanism. It's sorta on my "things to do when I have more time" list, along with other much-postponed items like writing FAQs for games which few other people care about.
So until I get more organised about keeping records of these things, I'll keep getting "surprise" periods. At least I have some hippy disposable sanitary towels in my desk drawer at college now.