What Causes Abdominal Pain And Tightness In Lower Abdomen Post Menopause?

I am a 55 yr old female, post menopause. I have abdominal pain - a low feeling of tightness and twinges on both left and right hand sides. The pain is mild and I have had it for over a month. Last week I had abdominal and transvaginal scans and both were fine. No cysts or other abnormalities. CA125 was 5.
I have a serum urea reading of 1.? (Not sure of the second figure) and I have an appointment for this in February.
No back ache, no blood in urine, no urinary problems I know of.