Benadryl can be given if you are *certain* that it is an insect bite and not a cat bite infection. Call your vet for the right dose for your cat. If you're not sure, or if he's acting at all sick or lethargic, he needs veterinary attention right away.
I agree with Jeannie, its best to take him to the vet. Jimmy had a swollen paw a few months back. When we took him to the vet, it was quite a simple and inexpensive procedure, and it gave him instant releif.
The vet drained off the poison with a syringe. Then she just prescribed antibiotics to stave off the infection. He was walking fine on it almost immediately, as soon as she had drained the poison.
We never did find out what caused it. Vet said it could have been a small bite or cut that had become infected.
I hope you will find your Kitty as easy to treat as Jimmy. But I'd say the sooner you go to the vet, the quicker it will all clear up.