A small lump or swelling under the jawline is most likely a swollen salivary gland or lymph node. Swelling of a salivary gland can have multiple causes, such as infection, inflammation related to an autoimmune disease or a blocked duct.
Are there other signs or symptoms present? When an infection is present in a salivary gland, there is usually fever and pain over the area. Swelling without much pain can be caused a small stone that blocks the flow of saliva out of the gland. The rash could be due to an irritation, infection or related to the existing condition. It would be best to see your doctor to rule out other causes. A complete medical and physical examination is important to determine the underlying cause.
Take care and keep us posted.