It is difficult to diagnose a
rash on net. This could be an eczema, dermatitis, sweat dermatitis, fungal
infection,
infection with stool, poor hygiene causing infection. If you have diabetes then this needs to be investigated as it can result in skin problems. After a course of antibiotics also it is possible to get both bacterial and fungal infection of skin. However it cannot be an allergic reaction to the antibiotic as that would be more general.