Welcome and thanks for posting your query to Healthcare Magic.
As per the query provided by you it appears to be closest possibility of "fungal infection" of the area. It appears as brownish to reddish marks anywhere body.
The main cause of this problem is because of growth of fungus. This fungus also grows due to heat, humidity and improper hygiene.
For the betterment follow the points below:
1. Take daily two time bath.
2. Wear cotton clothes and cotton undergarment.
3. Stay away from any condition of heat and humidity.
4. Do not scratch or rub the skin as it can lead to further aggravation.
5. You can apply anti-fungal creams over the affected area like "clotrimazole" or "terbinafine".
6. If any itching is present then you can take oral anti histamine
or levocetrizine under medical advise.
Your weight loss
do not appear to be correlated to the rash. Foer best of diagnosis you can meet a dermatologist
Hope these informations will help.
"With good health wishes"
Dr Sanjay Kumar Kanodia
MD (Dermatology & STDs)