This could from a number of causes including infection, inflammation, trauma, parasites, or cancer. There are a few clues you can get from the physical appearance of the lump and whether it is in the muscle, the skin or under the skin. However, the only way to conclusively determine what it is, is to have your veterinarian examine it.
The first step will be to do a fine needle aspirate. This is where a needle and syringe are used to suck a sample out of the lump. The sample is put on a slide and examined under the microscope. Depending on the results, it may be necessary to do a biopsy. This is where a sample of tissue is removed from the lump. The sample is usually sent to a specialist lab for examination. A biopsy is a better test because it examines a larger sample which has not been disrupted by forcing it through a needle.
The first step will be to do a fine needle aspirate. This is where a needle and syringe are used to suck a sample out of the lump. The sample is put on a slide and examined under the microscope. Depending on the results, it may be necessary to do a biopsy. This is where a sample of tissue is removed from the lump. The sample is usually sent to a specialist lab for examination. A biopsy is a better test because it examines a larger sample which has not been disrupted by forcing it through a needle.