What Causes Post Coital Bleeding For A Week After Intercourse?

I gave birth a year ago and got the Depo injection six weeks after giving birth. I had a small bleed a couple of days after I received it and then nothing for the next nine months. I stopped using Depo 7 weeks ago and since then I have had post coital bleeding the morning after sex, usually lasting for a week or more. What could be causing this?
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