Your baby isn't the only thing causing weight. You have more blood, fluid, the placenta. After I had my first after 4 days I lost 25 lbs. If you only gained 13 and you'll probably only gain at least 20lbs by the end you'll drop down to your normal weight very quickly.
Typically you lose 12lbs when you have the baby. Baby isnt all the weight you also lose the weight of the placenta and amniotic fluid