Hello,
From your symptoms, erythrasma needs to be ruled out. It is a chronic superficial infection of the intertriginous areas of the skin. It is true that groins are effected but the typical appearance is well-demarcated, brown-red macular patches. The skin has a wrinkled appearance with fine scales.
It has to be differentiated from contact dermatitis, psoriasis, sweat dermatitis and fungal infections which may mimic it. Wood lamp examination and gram staining are the tests for its diagnosis.
So nothing can be confirmed without examination. 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.
Your skin is just razor burnt, dehydrated and irritated. The reason the medication is ineffective is because you're most likely using it for a condition you don't have.
The skin in this area is very sensitive, as you know, and for first-time shavers the experience can be a little frightening. Yes, new and clean razors are safest, but they also shave closer to the skin, removing the protective layers and exposing your pores to more sweat, heat, bacteria, etc.
The best thing you can do is to let it heal on its own. No meds. Do not wash the area with anything but cool or warm (not hot) water when you shower, and avoid itching or tight clothing. If you must apply something for the sake of your sanity, pure aloe vera gel (without any additives) would be your best bet. Be patient. You should see improvement in a couple days.