A few days before and during your period, you might feel cramping and bloating in your abdomen. The cramps are caused by increased production of hormones. These hormones (called prostaglandins) cause the muscles of the uterus to contract. Many teens who have cramps also notice aching in the upper thighs along with lower back pain. Some also notice nausea, diarrhea, irritability, headaches, and fatigue, among other symptoms.