Hi
Since you are in a non-UK country, I am going to be a bit more specific than just sending you to the vet.
It is not possible for us to diagnose just from a little bit of hair loss and just from a picture.
However, I would recommended in view of the location you keep an eye out on whether the hair loss continues and starts to form a kind of V-shaped pattern on the back that is typical for mange/selnic mites (trixacarus caviae).
Treatment is with an ivermectin or selamectin based product at veterinary strength and not at preventative low dosed pet shop strength - the latter is useless for preventing an outbreak and promotes resistance; it is also not high dosed enough to get on top of mange mites in an acute outbreak. It is ultimately both long term cheaper and more effective to hit skin parasites hard with a vet strength product and to make sure that they are not coming back again.
Please be aware that you or your vet need to weigh your guinea pigs and be very careful to not overdose. That is the reason why we strongly recommend our members to see a vet for the correct dosage and good quality treatment. You need to treat at least 3 times at the product appropriate interval. It will take around 2 days for any skin treatment to be absorbed and the discomfort is shortly worse as the mites in the skin react to it. Itchiness will build up again when you come towards the second round but ideally you guinea pigs should be free from discomfort from two days later on. The third round is usually there to take care of any latecomers and prevent a comeback. In more advanced cases, more rounds of treatment will be necessary but you are hopefully on top of it quickly and can stop anything before it surpasses nuisance level.
You will need to treat all guinea pigs in contact with an infected guinea pig but the parasite is species specific.
There is a chance that this could have another cause, like for instance ringworm or just a tiny scab on the skin being scratched off. These things are often not quite textbook clear in the early stages. The next 2-3 days will show better what is going on. It is not pleasant for you to wait watch but but ultimately more effective and less uncomfortable when you can step in with the correct treatment at the ideal strength rather than home treat on spec and still end up at the vets weeks or months later with much more suffering for you piggies.
Here is some more information and further links:
New piggy problems: URI - ringworm - skin parasites