A period lasting 15 days followed by severe one-sided lower back and abdominal pain usually points to a problem in the uterus or ovary rather than ordinary period pain. At your age, this pattern often comes from a fibroid, adenomyosis or an ovarian cyst that has bled or twisted.
Next Steps
Since the pain is bad enough to stop you standing or sitting comfortably, please see a gynaecologist in person today rather than waiting for a video consult. A pelvic ultrasound will usually show whether a fibroid, cyst or thickened lining is behind both the long bleeding and the pain.
Health Tips
Go to the emergency department straight away if you develop fever, vomiting, dizziness or fainting, or if the pain keeps getting worse.