Itching in the buttocks is a typical phenomenon after a good spanking. The itching usually begins 8 to 12 hours after the spanking and is a sign of a healing process in damaged skin and muscle tissue.
The itching comes from decomposition products of dead cells (e.g. blood cells in a bruise). The remainders of these are carried away in the further healing process.
It typically itches more when the person sits down, which applies pressure to the tissue. Scratching or lotion does not help much because the source of the itching is not on the surface but in layers below the epidermis.
The end of the itching phase is a sign that the first phase of the healing is finished.
Itching in spanking novels
Post-spanking itching is mentioned in some spanking novels.