The back of the head raised a bag a little itchy see the following reasons: 1, the most common is the local sebaceous cysts, can cause local itching. The sebaceous glands can discharge sweat outside the body when they secrete sweat normally, but due to the blockage of sebaceous ducts, sweat cannot be discharged and will gradually increase in size, forming a local lump. 2. Consider local chemical allergies or mosquito bites that cause local skin allergic reactions, often with itchy skin surfaces, and if repeatedly scratched and itched, it will cause the local lump to increase in size, the allergic reaction will be enhanced, and the local skin will be more red and swollen. will also be more red and swollen.