The rash on my legs looks far worse but the rash on my lower abdomen is far far more itchy. Also I do not have any on my actual groin or genitalia which i would expect if it was due to washing boxer briefs with the wrong detergent.
It can be dermatitis (atopic, contact, allergic), heat rash or excessive sweating causing sweat dermatitis.
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 deodorant / cosmetics. You can also use a barrier cream with zinc oxide paste applied directly to wet skin after bathing. Take some antihistaminic lie Benadryl or Claritin for the itching.
If the rash persists then it will be best to get it evaluated from a dermatologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.