Most likely causes

• Pain from endometriosis often begins before the period and continues through it. It is usually felt on both sides of the pelvis, and low back pain can occur. - Berek & Novak's Gynecology, p. 587 area (Pain section).
• Pain severity does not match how extensive the disease is. Mild disease can hurt a lot. - Berek & Novak's Gynecology
• The uterosacral ligaments are the main pelvic source of chronic back pain. Endometriosis can involve them and cause pain in the sacrum. - Adams and Victor's Principles of Neurology, 12th Ed., "Back Pain from Visceral Disease"
• Rarely, cyclic swelling of endometrial tissue near the sciatic nerve or lumbosacral plexus causes leg pain, cramps or sciatica that follows the menstrual cycle. This is called catamenial sciatica. - Localization in Clinical Neurology, 8e; Adams and Victor
• Adenomyosis causes similar cramping and heavy or painful periods.

• This is a dilated pelvic venous plexus. It causes a dull, heavy pelvic ache that often spreads to the back and legs. It is usually worse after standing for long periods. The textbook notes its diagnostic criteria are not settled. - Berek & Novak's Gynecology

• Pelvic infection or a UTI can cause lower abdominal pain, back pain and sometimes fever or discharge. Infection can flare after menstruation because the cervix is slightly open then.

• Blood loss from your period can lower iron and ferritin. This can cause leg cramps and restless legs, fatigue and sometimes aching. Low magnesium, calcium or potassium can also cause cramps.

• A disc problem or lumbar radiculopathy can cause back pain with pain or cramps down the leg. It can seem to flare around your period because pain sensitivity rises and prostaglandins are active at that time.

Red flags: get seen urgently if you have any of these

• Fever or chills, or foul vaginal discharge
• Severe sudden pain, fainting, or a positive or possible pregnancy (ectopic pregnancy)
• Numbness in the groin or saddle area, trouble passing urine or stool, or progressive leg weakness
• One swollen, red or tender calf (possible DVT), especially if you use hormonal contraception
• Very heavy bleeding, or breathlessness and dizziness

What to do

1. See a doctor or gynecologist. Tell them when the pain starts, how long it lasts, and whether it repeats every cycle.
2. Expect these checks: a pelvic exam, a pregnancy test, a transvaginal ultrasound (it can show endometriomas and adenomyosis), a urine test, and bloods including CBC, ferritin and electrolytes. MRI and laparoscopy are used if endometriosis is suspected and the first tests are unclear.
3. For now:
   • An NSAID such as ibuprofen with food, unless you have ulcers, kidney disease or asthma triggered by NSAIDs.
   • A heating pad on the lower abdomen or back.
   • Gentle stretching and walking.
   • Keep a symptom diary of pain, leg symptoms and bleeding across a few cycles.

• Your age and whether you could be pregnant.
• Whether this happens every cycle, and how long it lasts.
• Whether you have fever, discharge, urinary symptoms, or heavy periods.
• Whether the leg symptoms are on one side.