A swollen bag on the back with soreness when pressed may be caused by sebaceous cysts, or it may be caused by lipomas, fibroids and so on. 1. Sebaceous cysts: there are many follicular sebaceous glands in the back, if usually sweat more or clean up in time, it may cause the sebum in the follicular sebaceous glands in the back to be stagnant, resulting in clogging, clogging will result in the contents of sebaceous glands more and more, protruding out of the surface of the skin to form lumps, and it may also cause tugging stimulation of the local subcutaneous nerve, resulting in a soreness sensation. 2. Fibroma, lipoma: both belong to the common clinical surface swelling, the reason for the formation is still unclear, may be related to personal constitution, heredity and other factors. If the lumps in the back area are soft and mobile, they tend to be lipomas; if they are hard and relatively less mobile, they tend to be fibromas. Both can cause compression of the subcutaneous nerves, resulting in the symptoms described above. There may be other reasons for the swelling of a bag in the back, and it is necessary to exclude whether it is lumbar bone tumor or lumbar spine tuberculosis and other diseases. It is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and then give targeted treatment or treatment under the guidance of the doctor.