What Causes Vaginal Bleeding Postmenopausal?

My mom broke her back about a month ago and had surgery to fix the break. She has been out of the hospital/rehab for about 2 weeks now and is doing an outpatient kind of rehab now. She recently started having vaginal bleeding, well more like spotting. Do you have any ideas why this is happening? She went through menopause about 7 years ago.