What Does An Oval-shaped Red Mark On Buttocks Indicate?

I noticed an oval-shaped red mark on my 2-year-old's bottom during her bath on Thursday morning. I thought she might have bumped herself in the tub as she is very active in there.But when I changed her diaper to put her down for her nap a few hours later, the red oval was still there. I touched it, and it felt hot, like a spider bite. The next day, the redness is fading into an almost waffle-pattern. My husband and I worry that it might be a tick bite bull's eye? We're perplexed by it. She hasn't been scratching and doesn't seem to notice it at all.