Bee

Bees are insects that like to hide in trees, and come out when you shake them! To catch one, you need to be quick. When you find a swarm, you must quickly take out your net, turn around, and hit A to swing at them. If you miss, you'll be stung though!

Cure
The only cure for bees stings is medicine that you can buy from Tom Nook's Shop. Alternatively, save your game, and quit. When you restart, the sting should be gone.