Hello,
I cannot confirm anything without examination but the possibilities of contact dermatitis, allergic reaction and axillary granular parakeratosis. It appears as brown- or red-crusted patches, papules, or plaques. I suggest you to get it evaluated from a dermatologist to confirm the diagnosis.
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 dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.