What Causes Spotting Followed By Period Inspite Of Being On Depo Shot?

I ve been on the depo shot for about a month now. I had a period that last three days and now I ve been spotting for about 5 days now. Its not enough to need a pantieliner or anything. I have the constant urge to pee all the time now. And its just Really annoying. Any suggestions on what Is causing this and about how long it will last?