Welcome and thanks for posting your query to HCM.
You are suffering from fungal infection
of the inter-crural areas known as Tinea cruris
This problem aroused because of exposure to heat and humidity and otherwise also due to continuous sweating i this area as is closed. These factors increases the propensity of fungus to grow. There are chances that if your partner is having similar kind of infection then you can also have contracted the same from him. So you should maintain proper hygiene with twice daily bath, wearing loose cotton clothes, changing your undergarments and general garments two times a day and maintain dry environment to prevent growth of fungus which will prevent recurrent/ further infection. You should take oral and topical treatment of antifungals for around 2-3weeks period under advice of your doctor.
With best health wishes,
Dr Sanjay K Kanodia