Making room for a baby, including a placenta and amniotic fluid, means that your internal organs will get gradually pushed and squashed around.
What’s more, the increase in lung capacity from above (see being short of breath), means you are being pushed from above and below.
The following animation from The Museum of Science and Industry in Chicago, demonstrates the movement of organs during pregnancy.
During pregnancy, this movement of the organs happens slowly and gradually. However, immediately after birth, there is suddenly space for the internal organs to drop back into again. It can be a strange sensation at first, although many mothers will not even notice it because they are so busy with their new baby.
Remember that your body has undergone huge changes. Go easy on yourself and take your time.