What Causes Lower Back Pain And Thickening Of Endometrial Wall?

I am 51 yrs old, post menopausal and recently had a CT Scan of my chest and abdomen. I have been experiencing intense lower back pain that wakes me up from sleep. The pain subsides during the day when up on my feet & sitting. The CT Scan findings included heterogeneous hypodensity within the uterus. I have had a polyp removed within the past 5 yrs that showed it was well on it s way to becoming cancer with the margins being clear of any pre-cancer cells. I am scheduled for a intravaginal sonogram next week. How serious can this finding be? Should I be concerned of the possible outcome? What can be the cause of the thickening of the uterus wall??