Why does my Cat keep going to litter box?

My cat keeps. Going back. And. Forth to litter. Box I. Took him to vet last week. He said. That. My cat was not conniption gave in a shot just in case of infection and inflammatory. Shot
Try

Clearly something is wrong with your cat. If your cat is constipated, give it canned Pumpkin (from the grocery store). You can also give your cat a tablespoon of milk / day to help with constipation. I would take your cat to the Vet to make sure that's what's going on. A qualified Vet can actually feel a cats belly and tell how much poop is backed up in your cat. 

Also stop feeding your cat dry cat food, as that will make your cat constipated.  Start feeding your cat wet cat food primarily.