Cats not getting along after hospital stay

Ok this is a new one for me. We had to take our oldest cat into the hospital last night for surgery, and brought her home today. Our issue is our youngest is now hissing at her, and doesn't seem at all happy. I find it funny because the roles were reversed when we brought the kitten home over a year ago. This is the first time they've had issues outside of the initial getting to know each other phase. And it was only one day that the cat was out of the house. Anyone else have this issue before? Thanks!

rjhall2 - It sounds to me that your older cat may have developed a "new smell" from being at hospital the that your kitten is now unfamiliar with. I would give it some time to see if the kitten adjusts.

Well there's tons of solutions that had to be used on your cat in the hospital, tons of smells she could of and probably did pick up.

This happened to me after I had my male cat neutered. My female cat absolutely hated him but their Doctor explained that, as far as my female cat is concerned, I just brought home a completely new cat. So the trick is to get the your female cat to start poooping and urinating. The more days go by with her scent being re-introduced into the environment, the less aggressive your male cat will be toward her. Eventually, they should go back to normal.