What Causes Uterus Walls To Thicken?

I am 60; started my changes about 10 yrs ago, since then i haven't had any vaginal bleeding or any symptoms except for some pain on my right side. since i was in my twenties i have IBS and possibly Chrones (maybe, never been conclusive)and fibroids(never bled) . I just went thru numerous examinations and health tests; everything came back normal. Went for a gynological exam and asked for the C25 blood test to check for ovarian cancer. it was suggested that i have a vaginal sonogram. i did and the Dr. now want to do a biopsy b/c she said that the walls of my uterus are thinkening. i had this done about 6/7 yrs ago and results were normal. She said that we will check again in 3 mos. Should i be concerned? Should i have the biopsy done sooner? Are there other causes for this besides cancer?