With no other symptoms it could possibly be...
Atrophic vaginitis
After menopause, levels of estrogen change dramatically and this can cause changes in vaginal tissues called atrophic vaginitis. A thin, watery discharge, sometimes blood-tinged, may sometimes occur, along with inflammation of the vagina and thinning and shrinking of the vaginal tissues. It only occurs in women who are post-menopausal or whose ovaries have been surgically removed. Symptoms may be aleviated with the use of topical estrogen creams.
Always worth a talk with GP or practice nurse.