What Causes Reoccurring Rashes On Left Side Of Buttocks And Waist Line?

I have a reaccuring rash that is usually a red patch of little bumps, sometimes up to a quater size area, sometimes as small as a dime size area. Also shows up in mulitple patches at times, but not always. Always shows up on the left side of my buttocks and waistline, never in the butt crack. Once the rash patches show up I will apply some cortizone creme or neosporane and within a 1 week or so, it crusts over and disappears, but seems to always leave like a discloration to the skin. Sometimes 1 rash will occur and then a few days later another patch will show up. Then other times I will go a couple of months without even getting a rash at all. It at first itches a bit when it shows up and then sometimes it also feels a little painful, but neither the itchy or painful is too much to handle.