If you are sure you cannot be pregnant, the other possibility is the hormonal
imbalance associated with peri-menopause
, considering your age. You should anyway exclude local causes as a precaution after trying to stop the bleeding with styptics and hormone supplements and get a pap smear
, a vaginal swab, trans-vaginal sonogram, urinalysis
and a complete hormonal profile. If local causes are negative, a course of hormones and supplements may be necessary. Hope this helps.