Its good to know that your PAP smears have been normal in the past.
Brownish discharge or bleeding persisting after age of 50years or occuring after menopause
should always be taken with suspicion and be completely investigated to rule out genital cancers.
Its not neccessary that your uterus
should show some mass for having cancer. Its only in advanced stages that positive ultrasound
findings come. Not trying to scare you but just making you understand that dont rely on ultrasound for any diagnosis. Final diagnosis can be made only after a tissue report which you would get only after undertaking an endometrial biopsy
Coming to the finding of heterogenous uterus , the report is very vague or may you havent mentioned the entire report above. Still it can be that the heterogenousity can be due to some cancerous process which is not making the layers of uterus appear clear. But to conclude cancer on ultrasound findings alone is not ethical and any cancer is diagnosed only after biopsy report.
If your brownish discharge is associated with foul smell , it can be due genital cancers.
Very rarely postmenopausal females have bleeding due to postmenopausal atrophy
of genital tissues which make the tissues fragile and cause bleeding.
Still to be absolutely sure , you should follow your gynecologist and get the biopsy done at the earliest.