Here are a few of links for info on ringworm (if it is that - it would be best if your vet could see her to confirm, so you know you are looking to treat the correct condition).

http://www.guinealynx.info/hairloss.html#fungus
http://www.copperlily.com/scuttlernet/ringworm.html
http://www.cavycrazyuk.com/fungal

Hopefully the vet will be able to advise you and give you something that is safe for a pregnant piggy. You need to get it treated or it's very likely her bubs will develop it too. Remember that griseofulvin, one of the most effective medications for ringworm in piggers, should never be given to a pregnant sow as it can harm the bubs.

I really wouldn't use any human medication on a guinea pig unless it is recommended by a cavy knowledgeable vet, especially not with your piggy being pregnant. Many medications and treatments are safe for us applied topically, but as a piggy will ingest whatever you put on it, you need to be extremely careful - even if you wait for it to sink in before you allow them to groom. It's just not worth risking.

Also see Peter Gurney's page on skin problems http://www.oginet.com/pgurney/skinp.htm . That does suggest a home treatment made of essential oils for mild fungal problems. Might be worth considering if you are unable to afford proper treatment. If she's pregnant, it would be far too stressful for her to do the full treatment now, especially with the necessary bathing, but you could make up a small amount to apply directly on the affected parts. It could keep it under control and would soothe the irritation. Provided she is housed inside, you can treat small areas without the need for bathing it off.

Remember ringworm is very contagious and they can pass it on to us, so be very careful with hygeine for yourself and between handling different pigs. Good luck with both at the vets.