What Causes Vaginal Bleeding And Stomach Pain While On Depo Shot?

Its going on 4 months that I had a baby and on my 40 day check up I was still bleeding. Just 3 weeks ago I started bleeding again even though I m on the depo shot. It lasted me a week and a half. Today when I went to use the restroom I noticed that I bleed threw my underwear and shorts but when I wiped myself there s no blood. I sometimes get stomach pain and look at food and get nauseous. And also it hurts to have intercourse with my husband. Why could this be?