Your daughter isn't just trying to get out of gym class — it's common for teenage girls to have unusually severe cramps and heavy periods. Don't just write an excuse note, however. Take her to see a medical professional. If there's nothing medically wrong, your doctor or nurse practitioner can prescribe ibuprofen for the pain, or even oral contraceptives to help with cramping and heavy blood flow. Although girls generally begin menstruating between ages eight and 13, if they're not menstruating by 16, see a health professional. Also check with your health care specialist if your daughter has very heavy periods. Heavy bleeding could result in anemia, or indicate a clotting disorder like Willebrand's disease.
As The Who would have said, the kids are all right. See the results of a survey on teen behavior at HowStuffWorks.