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Inner thigh soreness could be a result of skin inflammation
occurring as a result of irritation (clothes), excessive perspiration, fungal infection
, etc. We also need to rule out a sexually transmitted infection if you are sexually active.
I am unable to pinpoint a close possibility without an examination. Your physician / dermatologist
can examine the affected areas and suggest the cause/diagnosis. Please consider visiting his/her office at the earliest.
In the meanwhile keeping the area clean and dry is important. You can apply a moisturizer to soothe the irritation and use loose clothing / undergarments.
Hope this helps.