Hi. Development of such a large lump overnight without any symptoms is a very rare occurrence. The few possibilities I can think about are: Insect bite. Some insect bites may not cause symptoms, but as a reaction can cause swellings like you described. Another possibility I can think of is : there was a pre-existing lump like lipoma or sebaceous cyst, which might have increased in size to get noticed . The best way : you may post a good photograph here, our experts will tell you as a spot diagnosis./ visit a Surgeon who would examine and may advise some test if he thinks so and also will guide for further management .