I just thought I would share with you my experience with "lumps" on my animals:
First, my cat Lily had a dime-nickel sized moveable lump near her left hip. I'm in the medical field, so my first thought was to watch it and see if it got bigger....cancerous tumors are almost always rapidly growing--what makes the cells cancerous is that 1) the cells don't have the code in them to stop growing..cells are supposed to turn on, grow to the appropriate size for their function, and then turn back off, but cancer cells never turn off; and 2) they usually attach to other cells (even blood cells which distribute them around the body cause it to metastasize) causing those cells to change and become like they are....rapidly growing and not stopping. Anyway, I watched it and it never got any bigger, but I asked the vet about it anyway when she was in again. He took an aspiration biopsy (stuck a needle in and sucked any liquid (there wasn't any) along with some of the cells inside the mass into the needle) and looked at it under a microscope. He said it definitely wasn't cancerous (you can tell by looking at the cells) and that he felt it was most likely a foreign body reaction site (like getting a splinter, etc... under the skin and forming a lump because it doesn't belong there and is irritating the surrounding tissue--sort of the bodies way of confining the foreign body). As an alternative it could be a lipoma, which is a "tumor" (benign) made of fat cells--people get them all the time--I see them a lot. He said that if it were a foreign body it would eventually go away, but a lipoma would not. She still has the lump, and has had it for probably close to 2 years now, and it hasn't changed in size or shape, so I'm guessing it's probably a lipoma.
Second, when I first got Bella (my Yorkie), she had a little moveable lump near her left shoulder, and the breeder made sure to let me know that it was where she had her first vaccination and it would go away. It did go away, but she got another one each time she had her additional vaccinations. They have all gone away over a couple of weeks or so.
If your dog's lump is not near the injection site for a vaccine (which would be near a shoulder blade) and/or it has lasted for more than a couple of weeks or so, I would think it's probably a lipoma. Google "lipoma" and see what you think. (Emedicine.com is a really good people resource) Pets can get lipomas just like people can. Now, it the lump were growing at all, I would definitely get into someone who would do an aspiration biopsy (that's just how they biopsy superficial (close to the skin) masses). I have a feeling it's probably nothing to worry about at all. Just watch it!
