Thanks for writing here.
You are having perianal intertrigo
.Sweating ,friction with clothes can irritate skin leading to intertrigo.
Take regular bath after running.Use loose shorts while running.Keep area dry and clean.,you can use antifungal powder like candid
at day time.At night you can apply a steroid
antifungal cream like canddid b for 1 week.In case if there is no response it will be better to consult a dermatologist
Hope this will help you.Take care.