When i woke from general anesthetic from spinal surgery i woke with strong period cramps and then bled for 24hrs, 3 weeks early. not preg. this normal?

This is probably: Normal after a surgery. But see your doctor if this persists.

Not Much is Known: Or has been researched about this topic, although cytokine levels do increase during periods of surgical stress and these proteins prepare, or "confer receptivity, " of the endometrium (inner uterus lining) for menstruation. Together with the cascade of stress hormones released during surgery, altered period timing either early or late is likely to happen. Great question! and, don't worry!