The menstrual cycle of humans is complicated. It consists of a luteal and follicular phase, follows a roughly 21 day cycle, and has many effects beyond the shedding of the uterine lining. In addition to the cramps, bloating, hunger, exhaustion or nausea that some people deal with, many people also experience period related poop problems. Menstrual cycles are regulated by changing hormone levels, namely progesterone and estradiol (an estrogen), with some minor inputs from other biologically active molecules. One of these molecules is a prostaglandin that is released by the cells of the uterine lining when they die, that triggers the uterine contractions that expel the lining. The problem is that some of these prostaglandins escape the uterus and are detected by the smooth muscle cells of the large intestine, which are then triggered to contract. This results in, predictable, more frequent bowel movements during your period. It also turns out that the progesterone regulating your period can also effect your intestines, though contrarily to prostaglandins- by relaxing smooth muscle. This often manifests as fewer trips to the bathroom, or even constipation, during the luteal phase (from ovulation to just pre-menstruation) of the menstrual cycle.
The idea that birth control pills make the user gain weight has been floating around since the first appearance of contraceptive pills on the market in the 1950’s, but hasn’t been true for quite some time. Early contraceptive pills used only estrogen to prevent pregnancy, and they used it in massive quantities- initial pills had 10 mg of estrogen per daily pill. As the science of contraception developed however, it became obvious that lower doses of estrogen would accomplish the same effects (with fewer side effects), and the doses eventually dropped to the modern average of 35 µg per daily pill. While the high doses of estrogen were associated with weight gain in users, the modern amounts have dropped so low that studies find no relationship between combination birth control pills and the weight of their users.