Hi there & welcome to the forum as
@Poppy'sMum has suggested it could be ringworm & very contagious (pig to human,human to pig, pig to pig transmittable)
have a look at this link & it should help to decide
Ringworm: Hygiene And Pictures
but take both of your piggies to the vet as if it is ringworm they will both need treatment. Helen has already been tagged & she is very experienced on fungal / ringworm issues. i know she's a bit busy collecting one of her guineas after medical treatment today so it may take a while for her to respond.
if a culture is taken i understand that Ringworm is the only fungal condition that will show, other types of fungal will not.
i have had no experience of it in my own pigs but other members seem to have successful treatment with Itrafungol and/or Imaverol dips.
Get them to the vets for proper diagnosis though as it will need treatment if its a different type of fungal & not ringworm anyway. if i remember rightly it will probably look worse before it gets better after treatment. so be prepared)
oh & if you got them from a pet store keep© the vets bills & if it was from a certain well known chain store in the UK they are usually pretty good at paying the veterinary bills in these cases. Go into the place you bought them & tell them you bought unhealthy pigs (mention the animal welfare act) . they need treatment & you expect them to pay. i would suggest you don't let them take the girls back to treat them, but do it yourself - you know its been done & it will be less stressful for the girls.
Stress is also a trigger for illnesses in guineas, just the rehoming process is enough to knock there immune system for six for a few weeks & makes them susceptible to conditions such as this, mites, lice, upper respiratory infections, etc.
But dont be put off, she looks like a gorgeous little bundle

with the correct treatment & TLC she should recover fully in a few weeks
healing vibes from the girls & I. xx