What Causes Dark Brown Vaginal Discharge After Having Depo Shot?

I had the Depo shot the 15 of December during my period and I have had lots of Dark brown blood on toilet paper after wiping and spot bleeding ever since I have had blood test and doctor has not been very good with her info to me she has had to consult with a specialist and she has mentioned cancer but hasn t backed it up with info to me I have asked lots of question but she has said I have to have more test