It's possible that it's fungus but normally it would be other places before here, such as around the nose.
Do you have a "black light" or UV light? Most fungus will glow under one and you could check her that way to see if it does glow. Treating her with a "athlete's foot" powder in her dust ( 2 Tablespoons to 1 cup, will probably need to convert them into metric) won't hurt her if you want to treat her for fungus that way as well.
Does she itch there often? Does it seem to bother her?