Hello,
This can be due to yeast skin infection. The other possibility would be intertrigo. Most cases respond to over-the-counter products, which contain any of several basic ingredients: miconazole, tolnaftate, terbinafine,ketaconazole and clotrimazole.You can consult a pharmacist and get any cream or lotion containing these ingredients.
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.
This could be the start of eczema, psoriasis, etc.
While still possible that it was caused by a deodorant or soap, it's best to have it checked out.