4 year old having a skin rash on the buttocks. History of eczema. Could it be a yeast infection?

My 4-year-old son has had a weird looking patch right at the top of his buttcrack for a couple of weeks now. He has a history of having eczema . It does not seem to bother him at all or at least not that I can tell. He does still wear diapers. A few weeks ago, I tried bubble bath in his bath water. I didn t notice anything until a couple weeks ago and that was the last time I used the bubbles. The patch looks like a slightly raised area that is red and almost feels like a callous. It does have a couple areas where it appears that there are blood pixels coming to the surface of the skin but nothing has ever broken open. It does not ooze or anything. It sort of looks like goosebumped skin. I was thinking that it was just a patch of eczema but I am not sure since it is still there. Could it be a yeast infection? What else could it be?