Hi!
Please have your guinea pig vet checked for ringworm (the most contagious form of a fungal skin infection that can also transmit to humans and other species pets). Because exposure and infection has happened at the pet shop and you have in effect been sold - knowingly, I may add - damaged ware, your customer rights allow you to reclaim any vet cost resulting from this from the shop or ask them to cover any cost straight away. Please do NOT re-sell your piggy to the shop for the duration of the treatment as this is a trick so you have no rights or say in the quality of treatment and no handle to complain if something is going wrong.

This guide here informs you on your customer rights and how things are handled in various countries:
What to check for in new guinea pigs (vet checks, sexing, parasites, ringworm&URI)
Please DO NOT try home treating on spec and ending up paying a lot more without being able ro reclaim it by that time with a much more advanced cased and a real battle on your hands to get on top of the thousands of invisible long lived spores spread everywhere, not just the acute outbreak.
Ringworm is one of these things where you can't cut corners if you want to get on top of once and for all - you do it at your own peril unless you want more patches all over the piggy body and on yourself...
Good hygiene and treatment is key. A vet grade antifungal (and not just antibacterial) disinfectant is your best friend.
This guide here contains pictures of how a typical outbreak is running its due course when treated correctly. It also lists treatment products that work (cheap brioad spectrum pet shop products and creams are too weak and just another way for the shop to take your money) and what you need to do prevent the spores from spreading and causing more outbreaks in the months to come.
Please take the time to really read it and follow it, so you do not end with a never-ending nightmare:
Ringworm: Hygiene And Pictures