Hi,
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The symptoms could be suggestive of possible vaginal infections, cervix related problems, PID, endometriosis, adenomyosis, uterine fibroids, etc. The effect of Tamoxifen on uterus cannot be ruled out completely since you have been on it for prolonged duration. It is also difficult to comment on the exact cause without examining you and going ahead with further investigations. You should consider visiting your gynecologist/obstetrician for thorough evaluation and further investigations (especially pelvic examination and ultrasound scan) to determine the exact cause for your symptoms. Get started on appropriate treatment option (based on examination and investigation results).
Take care.