Cats skin crackling

Question
Hello.  A week ago tomorrow my 1-year old female cat was brought in to be spayed.  She was in first thing in the morning and came home later that afternoon.  She went through her course of pain medication and is currently not taking anything.  She is still very lethargic and sleeping a lot, seemingly using the bathroom and eating well though.

Just yesterday I noticed when I was holding and petting her that she has what feels like very tiny bubbles underneath her skin or that her skin is crackling when I pet her. It's all over her body and very strange.  She is moving around but when I hold and pet her it seems like she might be in some pain because she lets out shallow squeaks of a meow when I'm touching her.

Can you tell me what this crackling sound and or bubble sensation is with her skin?  It is the strangest thing.

Her sister cat that was brought home with her from the same litter, is going to go in next month for her spay procedure.  Ever since we brought the first cat, Carley, home from the vet, her sister, Mittens, will not have anything to do with her. Even a week later she growls at her and hisses when she sees the other cat.  Could this be because something is wrong with the cat that already had the procedure and has this strange crackling to her skin?  That is another question I have for you.

Please reply AsAP.  I do plan on calling out vet office to ask the same questions but since it's Sunday I figured I'd try this route first.

Thank you for your help/advice.
Crystal Plaster

Answer
This is called subcutaneous emphysema and it results from air being pumped under the skin because the incision is not completely sealed off. (Sometimes we will see this with chest injuries where air is escaping through the chest wall.)  The cat's activity pulls the skin open and pumps the air into the insicion.  When the incision heals, the bubbles under the skin will absorb and disappear.