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I would check closely around the foot and look for a source of pain like a cut or a thorn in the foot. If you don't see any, you can assume that the problem is with the leg and not the foot.
These little dogs are prone to patella problems (wiggly kneecaps) and this can cause some pain. If this is the cause, you can discuss using pain killers with your vet and also a surgical remedy.
I would also be concerned about the dog having an ACL problem. This is pretty painful and can cause some lameness in the hind leg. You don't see it as often in small dogs but it does happen.
If the pain lasts for more than 48 hours, you really need to see the vet to have it checked. Vets see dogs every day that have aggression issues and know how to deal with them. They might even want to anesthetize your dog so that they can do a thorough exam.
I hope this helps!
Let me know if you still have questions.