As you told your mother is post menopausal since 3 years but weighing 70 kg , which indicate that for that height she is overweight
is one of the cause to rule out .
As per USG findings she should undergo Endometrium biopsy
with histo- pathology ,torule out any cancer,or pre cancerous condition . All of above if all these are good , she should be under 6 momthly Usg guidence to see the endomeritrial -thickness.
If all these are within normal limit that is endomeritrium thikness less than 4 mm, not to werry .
you are welcome for further querry.