The bounce in your step has become a plod. Climbing stairs feels like summiting Mount Everest. Your brain's mired in fog. Whatever your personal energy crisis might be, it's time to act. "Fatigue that's new or dramatic, prolonged or unexplained can signal a serious medical problem like heart disease or anemia," says Lori Mosca, MD, director of preventive cardiology at NewYork-Presbyterian Hospital. "A lot of women hesitate to tell their doctors about low energy. But if you're feeling any new kind of tired, speak up." Your physician should check for the usual suspects, like insomnia, stress, sleep apnea, depression, and diabetes. If she rules those out, ask her to probe deeper for these often overlooked causes.