Hi, There can be multiple possibilities of yellow crust. You have not mentioned your age, sex, occupation and any recent change in routine. After reading your query there are two possibilities can be made: 1. Folliculitis: It is a Simple blockage and inflammation of the oil and hair root. It may cause dime sized knot underneath skin of leg. 2. Allergic rash: It may be due to contact sensitivity by some allergen in recent period. As this is painful so this possibility is less likely. You can apply antibiotic cream such as fucidin or mupirocin and take pain killer tablet such as paracetamol or diclofenac. Do not try to pinch pop or even touch the area as it may lead to secondary infection and further aggravation and spread. Take care.