Bump on kittens leg

A week or so ago I noticed my kitten Luna has a bump on her right leg where it bends.
It doesn't seem to bother her nor has it changed in size. She's already going to the vet on Saturday for some shots but I'm worried now. What could this bump be?

I'd send the photo to your vet and see if he or she can give you some advice.  Since you're going in anyway, it seems reasonable to do this.  Good luck.  I'm glad your kitty isn't in any pain.