We are very new to having a Guinea Pig and we are very stressed and worried about this bald spot appearing on her head. I don't even know what other details to include in the post but we haven't had her very long and we love her to death.
We know she is roughly 2 months old.
Hi and welcome
I am so very sorry that you are not having a smooth start.
Unfortunately, we cannot comment from just a picture but if there is white crust forming on the bald spot, then you are dealing most likely with ringworm (the most contagious and species jumping fungal skin infection), which looks to be the most likely issue you are dealing with. If not, it is more likely either hay or mange mites.
If in any doubt, please see a vet.
This link here explains more, including your customer right although you'd better ask what proof your pet shop is looking for now. Most likely a video containging the sales receipt and the date as well as you and the piggy and if needed any vet bill. Stay firm and demand to speak the manager.
New Guinea Pig Problems: Sexing & Pregnancy; URI, Ringworm & Parasites; Vet Checks & Customer Rights
For ringworm, please take the time to read and follow this guide here. Ringworm is mainly a hygiene problem rather than a serious illness although inadequate treatment can allow it to turn into one. You scant and undertreat at your own risk.
If you take our advice seriously, then you should be able to get on top of it once and for all instead of turning it into a never ending saga. We have had enough time in nearly 20 years to try all possible treatments and find all possible transmission angles the hard way - but also work out how to counter them efficiently.
Here is our step by step guide:
Ringworm: Hygiene, Care And Pictures
Please let us know what you are dealing with. We can answer all the little questions along the way and give you moral support. Please keep everything to this your dedicated ongoing support threat which we can let run for as long as needed. It really helps both sides to keep each case to just one support thread. Please bookmark this thread so you can pick it up easily whenever needed.
We do have a dedicated information collection for new owners into which all the most commonly encountered issues and questions have gone into as well as some practical and helpful step by step information with all the things that are relevant for new owners in their learning process.
It is yet another link for bookmarking:
Getting Started - Essential Information for New Owners
I hope that this helps you. Sorry that I don't have nicer news for you.