Thanks for the input everyone! Considering that this has been present for a week at most and is barely visible in photos (it's difficult to see even in person), I'm going to assume it's mild irritation or something - you know how pet-owner brain always makes you freak out and assume the worst! I'm going to rinse her hides off thoroughly today in plain water in case something in there got on her, and put some diluted betadine on that spot just to keep it clean - is vetericyn different from betadine? Should I keep both on hand? I'll start looking into a different disinfectant too, though it looks like F10 doesn't ship to California so I wonder if it's banned here for some reason. If it gets worse or hasn't cleared up after a week of betadine I'll revisit but hopefully it just goes away.