This lump should be examined by a veterinarian. Masses can be due to insect bites, trauma, infection (abscesses), foreign bodies (cactus or grass awns), cysts, and tumors. A fine needle aspirate will be done to determine the type of growth--this is a procedure where a small needle (same size as for vaccinations) is inserted into the lump to obtain cells for examination under the microscope. Bloodwork should not be necessary unless a mass removal is planned. Radiographs (x-rays) will not be useful; they are useful to identify bony tumors.
Any lumps on any part of the body of the dog should be taken seriously you should contact to your vet immediately and go xrays and fine needle aspirates and other blood tests