What Causes Bleeding Every Time I Have Sex After Delivering A Baby?

I had a baby 2 months ago, now when I have sex, I occasionally bleed. There is no pain just some blood. Sometimes there is only a little bit and just yesterday I bleed for a couple of hours...not alot just enough to see blood on my underwear. Could there be something wrong, or this normal since there is no pain and only lasts a short time and doesn't happen alot?