It seems that you might be having some abdominal muscle strain
due to hard hitting.
But any how any internal injury
in abdomen should be ruled out.
Consult your doctor and get examined.Ultra sound
of abdomen will give precise information about internal organs.
Meanwhile observe for bleeding from mouth or from stool or vomiting
and severity of pain, if you got any of these, go to ER and get investigated.
Ok and take care.