What could case a lump under my skin?

Lipoma: According to moy clinic:a lipoma is a slow-growing, fatty lump that's most often situated between your skin and the underlying muscle layer. Often a lipoma is easy to identify because it moves readily with slight finger pressure. It's doughy to touch and usually not tender. You may have more than one lipoma. Lipomas can occur at any age, but they're most often detected during middle age.