Is that the only place it is on? If this is the only area and there is no other involvment it usually does not suggest a reaction to something taken internally. But , if you have it elsewhere you better heed Dr. Chiro's advice. Interesting that it is in a intertigienous area where sweat and other mechanical forces may be playing a role. And, excuse my ignorance since I am still learning the jargon, but what is S1+?
Premier, as for prednisone. It has awful side effects especially if taken for a prolonged period of time. (diabetes, osteoperosis, cataracts, etc.) Some people need it acutely (such as breaking a severe asthma attack so they don't end up on the ventilator etc.). Shotterm it can cause weight gain, water retention, insomnia and even frank psycosis during the time you are taking it. It can suppress the adrenal glands if used more than two weeks at a time( or continuously.) It's used as a last resort though some specialties (derm) uses it like water to break any acute inciting skin rash but they do try to limit it to less than two weeks. In my body builders, because it is a catabolic hormone to some extent, we really try to avoid it if necessary and try to use topical and or non steroidal alternatives.
However, there are some diseases where there is no way around using it such as temporal arteritis (otherwise you go blind) and the initial stages of pulmonary sarcoidosis where you need to quickly get it under control (six months of steroids may actually be curative). Other diseases that historically used prednisone are getting away from it now (such as reumatoid arthritis) since now there are other non prednisone modalities and prednisone is rarely used by these guys.