It could be jock itch (fungus) But is more likely a chafing rash from sweat, etc. An STD hardly seems likely.
I suggest you keep the area clean (gently) and dry, using 1% hydrocortisone twice a day and powder after showering.
If that doesn't clear you up in 3 or 4 days, perhaps you should use some clotrimazole 1% creeam (OTC antifungal) and arrange to see a doctor.
Best.
Dr. Rockoff
Dr. Rockoff
http://www.randburkey.netfirms.com
***@****