Hello,
Eczema is a form of chronic dermatitis (rash).Allergic reaction is one of the important causes of eczema.It can occur anywhere on skin even genitalia.If you could elaborate more on the symptoms and the type of rash that you are having,it will be easier to chalk out the differential diagnosis.Other conditions that may involve this area are fungal infections(ringworm,tinea capitis),psoriasis,contact and sweat dermatitis and intertrigo.
Hope it helps.Take care and pls do keep me posted on how you are doing or if you have any additional doubts.
Does it appear dry and scaly? Does it itch?
Eczema can pop up anywhere but if you believe the doctors it typically spares that region. But Jock itch is a possibility. Need more info.
Try jock itch cream first. If it itches and has a border.