Hello,
Without examination,confirmation of a diagnosis is tough but it can be contact dermatitis,rash due to friction rub with clothes, dermatitis (atopic, contact, allergic),excessive sweating causing sweat dermatitis or balanitis(due to an underlying infection (bacterial and fungal as well as other organisms) .
You may try calamine lotion on the rash.You should also maintain hygiene and try to keep the area clean and dry, wear loose-fitting cotton clothing. Also use an antibacterial soap or mild soap like dove.Avoid any kind of cosmetics.You can also use a barrier cream with zinc oxide paste or calamine lotion or a mild steroid cream like dermacort on the rash.
If the rash persists then it will be best to get it evaluated from a dermatologist.
Hope it helps.Take care and pls do keep me posted on how you are doing or in case you have any additional doubts.Kind regards.
I think it might just be a rash, and maybe not at all related to the sex.