Guinea pigs/ skin problems?

The others are right, it's mites. You need to have her treated. When my guineas first got it, we had no idea what it was, thought it would go away, and before we knew it, one of them was half bald. Poor little girl. You can get a powder to put into water (I think the first answere already gave you he name. I couldn't be sure, since it's a long time ago with me, and I'm in Germany, so the name could/might be different, which is why I am not sure of the name), and than you will need to bath all three of your guineas with that. Our vet told us to sit them in a bucket, so they wouldn't be able to jump out (hahahahaha! What a good joke! you would have thought they were members of the olympic team, they jumped so high and far!) and to get a new sponge, get it real full with the bath and rub the bath into the fur softly, but firmly, against the line the hair is groing on your guinea, so as to be sure that the roots of all the hair is really getting a good dose. It worked wonders, really. Guineas were good as new, shiny hair, no more bald patches, and in time, they forgave the owners for the breach of discretion and taking the liberty of BATHING THEM! Horror of horrors! As to the owners: Great treatment. No mites, no shiny hair, but also no bald patches. Scratches on the arms. Got frequently asked if they had a new and particularly feisty cat. Needed to wash clothes right after treating the piggies. Bathroom carpet: Great condition after treatment. No mites, no bald patches. A slight problem with odor, and it never had shiny hair after (on the other hand, it wasn't particular shiny before, either...). ;)) Jokes aside: its a relatively easy treatment that works wonders. Our vet told us to be sure they wouldn't swallow it or get it into their ears, but mine just hated being bathed sooooo much that "taking care not to get it in there" was not a priority. Stopping them from killing themselves by jumping out wherever they saw a chance was. We ended up sitting on the floor while bathing them. Hope yours are a little mor compatible with the treatment! Good Luck!

This is a common condition in guinea pigs. If she just had her babies, this could be a cause of stress from labor. I have seen this condition in books, so do not be worried. Clean the place where her skin is irritated with something as simple as water. See or call a vet as soon as you can. It also could be mites or fleas.