Check her skin for lice or mites. If this doesn't clear up soon, I would see a vet about it. My RB pig Taffy had the same thing and the bugs weren't visible on his skin unless you saw it through a microscope.
It doesn't sound like an allergic reaction, but what kind of bedding are you using? Wood shavings do sometimes carry mites or lice inside them, so she mights have gotten some sort of bug from there.
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