What is a lump on your lower back but it doesn't hurt?

Cant tell: It could be cyst, fatty tumour- lipoma or other rarer conditions. The only way to tell is have a dermatologist see it and examine the rest of your body for any other suspicious lesions.

Simple vs serious: Our bodies always have things that grow and change. Skin, fat, muscle, lymph nodes, bones, can change shape, get larger, more prominent due to many factors. The vast majority are not cancer but simply unusual growths like fatty tumors, cysts, skin tags, moles, etc. Most cause no symptoms and can be watched. If they cause symptoms or change colors, bleed, bruise or hurt, then see your doctor.