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The description suggests it can be eczema or a fungal infection
, which is common at this site. You wil have to visit a dermatologist
for a confirm diagnosis & treatment.
Keep the area clean, dry, avoid scratching. The marks usualy fades with time. You can also try homeopathy
for the same. You should also check your blood sugar levels, and thyroid
levels , as they are often associated with similar skin condition.