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There could be two causes for this type of pain:
1) Endometriosis. Although the lap did not show endometriosis, very often, very minor seedlings could be present which is not visible to the naked eye. You can try taking a course of birth control pills for 3 months - the pain decreases, then you will know that it is due to endometriosis.
2) Pressure over the nerves to the abdomen. These nerves arise at the spinal cord. Pressure or compression can occur due to a slipped vertebral disc or arthritis of the spinal bones. An MRI will help confirm or rule out this cause.
I hope this helps.
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