problem with brno chair  

Hello,
Well i have a problem with my brno chair . There is a slight bump on the backside and i don't know how it happend .
Five days ago i placed the chair in another apartment and it stayed there untouched until today . Today morning i went there to take my chair back and i noticed this bump on the backside . It is slight , it isn't very noticeable and that's why i cant take a good pic of it . I don't know if it damage or normal wear and tear . This chair is from 2003 but it has been used rarely . Maybe i have to take it for repair.
So i would like to know if anybody had ever have the same problem with his chair ?