A few weeks ago, Dr. Jen Gunter began hearing from people on Twitter who had experienced something strange after receiving a COVID-19 vaccine: Their periods were unusual. Some reported menstruating early, right after receiving the shot. Others experienced unusually heavy bleeding. At first, Gunter, a gynecologist and righteous slayer of health misinformation (and the subject of a profile by my colleague Maddie Oatman) suspected the symptoms were the result of pandemic strain rather than the shot itself—stress is a known cause of menstrual cycle irregularity.