Relax! Your baby is in all probability OK. Kidney
and other deeper injuries too are unlikely with a kid punch. However you could have bruising to the ribs and the intercostal muscles.
What you should do is to take good rest. Avoid any activity that puts stress
on the muscles of your back.
Ask your partner to give you some hot fomentation to the region and apply some pain balm to the area.
If you don't feel better with this regimen in three days, consult your OBG..