What Causes Brown Vaginal Spotting Post Menopause?

Hello i am 70 postmenopausal no hysterectomy,active and have been healthy. all my pap test have been normal. Recently i was having itching an burning when urinating, went to Dr, was tested and culture done of urine, he said i had pus, blood in urine gave me antibiotics to take 10 days, drink lots water. sent culture to lab, all came back normal no bacterial growth in culture and said stop abtibiotics i had taken for two days. symptoms went away , i have noticed an occasional very small light brown spot on my underware after urinating, this has happened off and on for about a month, maybe a total of 3-4 times , there is no discomfort, pain or fever. What could possibly cause this