Candy ~

If the Vet can't find a wound or penetration anywhere on the paw ~ Suggest considering a broken part...

Shadow ( RB) *BROKE* her rear, outside TOE one night - stubbed it on something. She hobbled around and whined like she'd damaged her back or hip. Vet looked and prodded all the major structures and said "I bet she injured just that paw; my guess is a broken toe. Let's take a picture."

Sure enough - One Well Broken Toe.
Wrapped her paw and ankle joint and off she hobbled... for about two or three weeks.

She played the injury to the hilt - until the tiny sized (at the time) Boots Kat started to take advantage of the situation.
Boots was thoroughly enjoying pestering Shadow ~
Until Shadow had had enough one night - Boots had almost forgotten how fast he needed to run to stay out in front .

Hope the Vet Visit gets to the bottom of the problem!