Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Pinky has been non-weight bearing lame on his right rear leg with swelling at the knee joint for the past three days, and now has edema just above his heel. I understand that you cannot see any evidence of a wound or redness/discoloration.
Is he running a fever?
Does he allow you to touch his leg and move it through its range of motions without complaint?
Is the swelling at his knee soft or firm?
Does the swelling at his knee appear to be in the joint or around the joint?
Did he suddenly come up lame or was it a slower progression from slightly lame to non-weight bearing lame?
What was he doing before he became lame?
If you pinch his toes on that leg does he feel it?
My primary concerns with this history would be:
1) A puncture wound that has healed over but led to an infection deep in the tissues. Usually those dogs are running a fever, or at least the affected area would be red and "hot" to the touch. These pups are usually quite painful too and reluctant for us to touch the area. These dogs may feel poorly and are lethargic and may not eat very well depending upon the systemic effects of the infection.
2) A tumor of one of the bones or cartilage of the knee joint, the lymph node near the knee joint, or the muscle that is now affecting blood or lymphatic circulation. These dogs may or may not be painful with palpation, sometimes we must palpate a particular spot very firmly to get a pain response. They are not running a fever and seem otherwise fine for the most part. These pups often have an on and off lameness which eventually becomes non-weight bearing.
3) Damage to one of the ligaments or muscles between the knee and the hock/foot. These dogs aren't particularly painful, but when they attempt to bear weight the leg cannot hold their weight properly, the leg may give or collapse completely. They usually become lame suddenly. They aren't painful as much as they are weak. They won't run a fever and seem to feel fine otherwise.
Your fellow needs to see a veterinarian for a thorough examination and radiographs of the knee, possibly an aspirate of the swelling. Further diagnostics and treatment will be based upon that.
If your fellow is eating and drinking I recommend keeping him quiet and having him seem by his veterinarian tomorrow. You can alternate apply hot and cold compresses to the swollen area for 10 minutes at a time several times today if it isn't too painful for him. This may reduce swelling and increase comfort.
Although this doesn't sound like an emergency it is something that needs prompt attention. If this is an infection we need to start effective antibiotics quickly and if this is a tumor the quicker we address it the less time it has to metastasize (spread to distant sites in the body).
Please let me know if you have any further questions.