I'm pretty sure I have loooow magnesium and it gets worse near my period and I lose my appetite and gag on everything I eat. Bowels are loose but all I'm eating is salmon, sweet potato and raw carrots last couple days. The right amount of magnesium shouldn't cause you to be runny. If it does lower the dose until you find a level that keeps you regular. I would think a little should be taken regardless of diarrhea from another cause as...couldn't you be more deficient because of it?
I also find that I lose my appetite near my period, and crave dark chocolate, recently. Over the last 6 mnoths I also have prolonged muscle soreness after exercising. I have heard many women say that they crave soymilk before their period. (Or, as for me, dark chocolate . I wonder if this is the body's way of trying to get more magnesium to reduce uterine cramping, as soybeans are one of hte top 10 magnesium-rich foods. In any case it is interesting to hear your testimony about this.