What can cause a girl to discharge if she doesn't have a std and she's on the depo-provera birth control shot?

Leukorrhea: Pre-teen and teen girls usually have a clear to milky colored vaginal discharge. It is not a large amount, but enough to soil the underwear and sometimes cause some mild irritation to the outer vaginal area. Most girls can tell if the amount, smell, color, symptoms change. If the discharge changes then it could be an std or just due to the Depo-Provera shot.