My kitten got spayed about a month ago and she developed swelling. I freaked out! In humans you relate swelling to infection. However, after reading around online I found it's normal for kittens to have swelling after surgery. It comes and goes after a week or so. I will admit your kitten looks a little more swollen than mine though. I ended up taking my kitten in to the vet because her incision was really red and she had undone a stitch. To check for infection the vet checked her temperature and felt the wound a bit (at that time she already had a swollen bump and the vet said she seemed fine). I'd make sure she leaves the wound alone (my kitten didn't so she had a cone for a week), make sure she is eating and going to the bathroom, and if she seems okay otherwise I'd probably wait before going to the vet. However, I agree with Gayef, you know your cat best!!