As you have given the photo , it clearly shows that it's a fungal Infection.it may spread easily to other side of your thighs too.
Next Steps
Keep the affected area dry and clean . Due to excessive sweating may be the reason but many more reasons are there to get infected.
Health Tips
Use mixture of pure turmeric powder with little amount of rose water for external application twice or thrice a day at the Infected area. Still having any doubts contact me personally