PLEASE, switch back to the original food right away. If after a few days you are still seeing a problem, further fine tuning of the diet will be necessary.
Some cats never get used to certain foods, so the sooner you switch back to the food that didn't cause any problems for your kitten, the better.
I don't know what's in the Wellness kitten formula, but in the adult formula I counted seven different grains if I remember correctly. Way too much for many cats.
Wash your kitten's behind very gently with soft, wet white bathroom tissue (don't use colored tissue) once or twice a day and pat dry with dry tissue. If you call your vet they can give you a tube of Animax cream or some other ointment that will help heal the sore, red area very quickly.