I just for the first time picked up my cats kittens, they are 8 days old. I've watched them lots and never noticed nothing wrong with anyone of them, but I picked up my "favorite" about an hour ago and noticed its paw looked really swollen on top. I never noticed this in all the times I've watched/checked on them... what could this be caused from? Has this happened to anyone else?
There are a total of four kittens, they stay in the corner of my closet and the other pets in the house do not have any access at all to them.

Are there any teeth marks on the paw? Do you have a vet to call or take the kitten to?

Does it seem to cause pain? Is there any chance of getting a vet to look at it?

It's possible mom accidentally scratched or bit it. The other kittens don't really have the ability to cause any sort of injury.

Check the entire bottom of the closet CAREFULLY for any exposed nails, screws, staples, or splinters. It's possible something there scratched him.

Hopefully, it's just a minor scratch that will clear itself up. Unfortunately, kittens can get sick easily if things go untreated. If at all possible, at least talk to a vet (if you can't afford to take him in to see one) and see if there have any ideas and possible treatments.

It could be injury, birth defect, genetic anomaly, or who knows what else. Injury is the easiest to rule out.