That’s definitely an insect sting!!!
I’ve pasted some info below for you (the ice pack idea might help soothe your cat).
If your cat has been stung by a bee and the stinger is still there, you’ll need to remove it first. It’s best not to use tweezers, as you might accidentally squeeze more venom out of the sac if it's still attached. Instead, use something flat and firm, like a credit card, and gently scrape across the skin to flick the stinger out. Get someone to help you if possible, as your kitty might not be the most cooperative patient…
To soothe a bee sting, you can apply a paste made of bicarbonate of soda and water (bee venom is acidic, so this helps neutralise it). For a wasp or hornet sting, which is alkaline, you should use a bit of vinegar or lemon juice. If the area is swollen, use an ice pack (or a bag of frozen veg): the cold will help bring the swelling down and ease the pain. No matter what caused the sting or bite (wasps, but also mosquitoes, spiders, etc.), make sure you disinfect the area.
Some stings can look quite nasty, especially when they cause swelling, but they aren't usually serious—everything should be back to normal within a few hours. However, you need to be very careful if your pet has been stung in the mouth or throat, or if you notice swelling in those areas, as it can cause breathing difficulties. In these cases, you need to get to the vet immediately.
The same applies if your cat has an allergic reaction or has been stung multiple times. Be on the lookout for signs like struggling to breathe, excessive drooling, dizziness, vomiting, or diarrhoea. If you see any of these, get your cat to the vet or an out-of-hours vet straight away so they can give them the right treatment.
