Hi, Contact dermatitis or skin inflammation is due to adverse reaction to something that is allergic to your skin. This may include chemicals used in bathing soap, shower gel or detergent (used to wash the undergarments). It can also happen due to thighs rubbing each other, which is quite common in a person with fatty thighs. The rubbing of skins of inner thighs result in irritation (called intertrigo) that causes redness and inflammation of the skin. Allergic food and/or medication can also substantially cause rashes on inner thighs along with other sites of the body.  Psoriasis, impetigo and eczema are few of the diseases that are known to produce rashes or skin eruption. Kindly consult Dermatologist...