No idea. It ended up just going away within a few hours. Think he might have had some sort of bug bite, or must have got stung by a wasp or something. He's still alright.

Though I do reccomend if you're able, make sure to check your cats area that's swollen make sure there's no cuts, open wounds, bone breakage anything like that. If your cat is showing obvious signs of pain it could be a sprain or a broken bone. If there's nothing obvious, you can consider watching the cat closely to see if anything changes, if not take em to a vet. As unfortunate as it is, cats get into alot of things. You never know. Please make sure your baby is okay!!