CT scan shows rectal dilation

[HutchiK]

I have had pelvic pain and cramping for 7 weeks. Ultrasound was clear but CT scan showed an 8.5cm dilation. Colonoscopy was unremarkable with 3 small polyps biopsied. Nothing to explain the dilation. Rectal monometry was normal. Bowel function has been normal. I am a 7 year breast cancer survivor. Pain is predominantly in lower right back. Sometimes severe. Any speculation on what could be going on? I'm a 53 year old post menopausal woman.