Thanks for posting your query.
You are most likely suffering from Tinea cruris
which is getting worse after sweat secretion.
Presence of itching in groin along with soreness suggests the diagnosis.
You should consult with internal medicine specialist / dermatologist
and should go for thorough check up.
You should go for KOH mount preparation of skin scrapings from sctrum and groin area.
You should apply antifungal ointment along with oral preparation of antifungal medicines.
You should wear soft cotton cloths.
Avoid soap application.
Dr. Mayank Bhargava