Hello, welcome to Healthcare Magic.
The most probable diagnosis by reading your query appears to be fungal infection
known as Tinea cruris
which appears as circular ring like structures which are dark at first and then have central clearing later on.
For the treatment part you should take oral and topical treatment of antifungals for around 2-3weeks period under advice of your doctor.
This problem arises by exposure to heat and humidity. Both of these things increases the propensity of fungus to grow. 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 maintaining a dry environment to prevent growth of fungus which will prevent recurrent/ further infection.
If the lesions becomes more dark or reddish or purplish- then you should also take the possibility of purpura
which are red spots and would be definitely needing medical attention
With best health wishes,
Dr Sanjay K Kanodia