BIOLOGY | MEDICAL | HEALTH on Instagram: "During your period, the uterus contracts to shed its inner lining (endometrium). These contractions are triggered by chemicals called prostaglandins. Higher prostaglandin levels = stronger contractions = more pain. As the uterus tightens, it can temporarily reduce blood flow and oxygen to the uterine muscles. This lack of oxygen causes pain, pressure, and cramping, often felt in the lower abdomen, lower back, or thighs. Once the lining is expelled and pr