It sounds as if your dog has an infected pad or something is stuck in it or between the pads/toes. If he will not allow you to feel around the pad then he should be seen by the vet asap. He is suffering with pain. Good luck!
There is not enough information here to help you. What breed dog? Age? Sex? when did this start happening? Is the nipple on the same side as the leg? Is the swelling under the teet itself or in the general area? Is it sore or warm to the touch?
They get swollen and full, and tender to the touch!
It could have hurt him a little or it might of mean he's not interested in flirting, be sure he doesn't have a girlfriend.
Red, swollen, warm to the touch, tender.
Touch it...
Information keeps people in touch
The only way to correctly diagnose the problem is to consult the doctor
He may have Joint Ill.
Bug bite/sting?
it will be tender to the touch, red, swollen and possibly seeping pus...
If it is pussing or it hurts to the touch after healing. See also swollen.
If its been swollen, discolored (red/purple), or itchy for more than 2-3 weeks. Or if the drainage from the piercing is a dark yellow/greenish color. It could just be irritated if its red and swollen. Go check with your piercer to be sure.