I've always had my hunches.... When DS1 was about 12 months old I suddenly became very sick to my stomach for a few days with vomiting. Then a month later had my first pp period.
Now DS2 is 9 months old and out of the blue this weekend I got incredibly sick with vomiting again. There were a few tinges of blood in the CM as well, but not a full period.
I think this is my body's way of getting the ovulation going again. My PMS symptoms have always included nausea (but not vomiting) so I think it's related.
But I would LOVE to hear of anyone else who's had these same symptoms upon return of menstrual cycles. Anyone??