My dog was under my truck and i didnt see it but didnt get hurt a month later a squshy thing started growin?

my dog was under the truck and i was backin up and kind of hit him he couldnt walk for several of days but then got better then a month ago i notice somthing from the side of his stomach was poking out i didnt worry to much i thought it would go away but then it started growin it looks like a sack of water and gooshy too soo i would like to know what could it be one of my husband friends told him it could be a tumor or a blood shatter somethin like so if u could please help me with that