Welcome to HCM
First of all I would like to inform you that intramuscular injection
in gluteal region should be avoided specially in children not only injection at this area is works slowly(due to gluteal fat)but also there is a possibility of damaging Sciatic nerve
present at the region and it causes severe pain
.We see so many cases where baby come to clinic with intramuscular injection causing severe pain like your baby.So now a days intramuscular injection at back is avoided.
Now I advise you to go to your pediatrician
for check up and do the needful.