Period flu mimics actual flu, minus the virus. You might feel achy all over, run-down, slightly feverish-feeling without a real fever, nauseous, headachey, or just deeply foggy and exhausted. Some people get chills, joint stiffness, digestive weirdness like bloating or loose stools, and a heavy, low-energy mood. It usually creeps in a day or two before your period starts and tapers off once you're a day or two in. The pattern is the giveaway, if it shows up like clockwork with your cycle, that's period flu territory. If symptoms feel severe or you're spiking an actual fever, that's not just your cycle, please check in with a doctor.