What Causes Body Pain After A Child Birth?

I recently had a baby 3 weeks ago and my body has hurt ever since my knees, hips, and just starting the other day my mid back, then last night my right arm is now killing me. What could this all mean I have been having to live off of advil. oh I`m 5`3" and 150lbs I have lost 20lbs so far from my pregnancy. I was 125lbs before pregnancy.