sounds like a concussion to me...brains bang around during a concussion..they get bruised..it takes a little time for things to heal and settle down which is why you are having some delayed reactions.

i would keep your dr informed and just be careful to not bang thing around for a while. if things get worse or change significantly i'd let them no ASAP but otherwise i'd just keep cursing the possum.

and not for nothing...my one dog's personal favorite is groundhogs. we got to a dog park...a totally fenced in dog park. last spring he killed 5 of them inside the dog park. big ones. made me even crazier than i was before. made my blood pressure even higher than it was. didn't seem to bother him at all (he did get bit 4 times by one but he wore those bites like a badge of honor).

my other boy (the one in the picture) couldn't kill a dead animal if it landed on his feet. but produce...that he can kill...damn fool dog loves to steal tomatoes out of my garden, and avacodos, peaches, apples, plums etc off of the counter.