Thanks for the info. That helps me narrow it down a bit.
There are a number of possible causes for the bump. Given that your dog is relatively young and healthy otherwise, hopefully what you are dealing with is a sebaceous cyst. Sebaceous cysts are common skin growths in dogs, and they often form firm, round masses under the skin that can appear gray, blue-black, pink, or a combination of these. These cysts only involve the skin (i.e. they are not attached to the underlying muscle).
These cysts are caused by a buildup of a normal oily substance called sebum, which is secreted by sebaceous glands deep in your dog's skin. The sebum normally helps keep the skin lubricated. If one of the ducts that transports sebum to the surface of the skin gets clogged, the sebum builds up underneath and results in a cyst. Usually these are non-painful unless they are very large, which makes me think that the mass you are seeing may be something else, or the limp you are noticing in your pet is unrelated to the mass. Sebaceous cysts can rupture, and a whitish or brown cheese-like material will drain. Occasionally these cysts will resolve on their own. Dogs who get sebaceous cysts often end up developing them in multiple locations. They can also be removed surgically, but it is usually considered cosmetic.
The other possibilities for the mass include a fungal granuloma (this is a congregation of inflammatory cells and fungal organisms - usually dogs with this have other signs of illness), lipoma (accumulation of fat cells within or just under the skin), parasitic disease (not likely), or tumor. Unfortunately the only way to tell the difference between these is to have the mass examined by a veterinarian who will likely perform a fine needle aspirate. This involves sampling the mass with a very small needle, and examining the contents under the microscope. It can be done while you wait, and the dog does not need to be sedated. A full physical exam can also reveal if there are other masses present, if the dog's lymph nodes are enlarged (which would support an infectious cause or tumor), as well as try to determine why your dog is lame. If the mass is big enough that it is pushing on one of the nerves that supplies the affected leg, it could be causing the lameness, but there are a number of other potential causes as well. I would recommend making an appointment with your veterinarian to have your dog examined within the next day or two if possible. In the meantime, keep your dog rested to help minimize any pain he is experiencing and in case he has a muscle or ligament injury associated with his lameness. Also, try to leave the mass alone as much as possible until you have a chance to get is assessed. Although unlikely, there is one type of skin tumor (called a mast cell tumor) that can release histamine and cause an almost anyphylactic-like reaction in your pet if it is poked or prodded too much. It is best to find out exactly what it is first.
Let me know if you have any other questions about it. Best of luck.
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The information contained in this post is not meant to be a diagnosis, and should not take the place of an examination of your pet by a veterinarian.