Your description does not actually concern me much for a staph infection. It sounds more like it may be an area of contact irritation or eczema.
If the area continues to spread you should have it checked out as it may still be a staph or other bacterial infection.
Treatment for eczema and contact irritation (contact dermatitis) is a topical steroid cream such as hydrocortisone. Hydrocortisone is available without prescription but is fairly weak. If it is not getting better your doctor can verify that the area is such a condition and if you need a stronger, prescription cream.