Lumps, bumps, or growths under your skin aren’t uncommon. It’s completely normal to have one or more of these throughout your life. A lump can form under your skin for many reasons. Often, lumps are benign (harmless). Cancerous lumps that can be felt from the outside of your body can appear in the breast, testicle, or neck, but also in the arms and legs. One type of cancerous lump that can form almost anywhere in the body is called adult soft tissue sarcoma. Adult soft tissue sarcoma is rare.