your tummy went through a lot of stretching during pregnancy, and within minutes, POOF, baby's out.
It is very normal, as your body will take time to adjust itself. Please dont't expect yourself to be pre-pregnancy size so quickly. Rest well, drink lots of water, and eat well.
If you are breastfeeding, continue to do so as it will help to shrink the tummy. I remember, by the time I got out of the hospital, my tummy shranked already, but stretch marks and dark line still there