Hi!
Please step in with syringe feeding fibre ASAP and see a vet as soon as possible! Loss of appetite is always a danger sign that needs attending quickly; you cannot afford to wait days to make up your mind whether to see a vet. If a piggy of yours is ill soon after your arrival, it is most likely that this has already been going on during your absence, so by dithering you are losing more valuable time.
The heat may contribute to the loss of appetite, but it is not causing it. Dehydration is usually caused by an underlying issue if your piggies have constant access to fresh and regularly refreshed water bottles and fresh, juicy veg.
Weigh your piggy daily at the same instead of the regular weekly weigh-in to monitor the food intake. Be aware that hay is making 80% of the daily food intake, and that you cannot control it any other way, certainly not by watching a piggy chew on crud or nibble on a few stalks. Hay is often the food group that is dropped first when there is any discomfort or pain. Huge swings of temperature can also have an impact, but normally only on guinea pigs who have pre-existing underlying issues.
List Of Life And Death Out-of-hours Emergencies
Not Eating, Weight Loss Over 50g And The Importance Of Syringe Feeding Fibre
Complete Syringe Feeding Guide
The Importance Of Weighing - Ideal Weight / Overweight / Underweight
Here is a collection of tips on how you can keep your piggy as cool and comfortable as possible:
Hot Weather Management And Heat Strokes
There are lots of reasons for the symptoms that you are reporting. All we can do is guess as much as you do. Only a hands-on examination by an experienced vet will tell. Until then, you need to concentrate on keeping your piggy alive until any medication is kicking in. The sooner you can see a vet, the better!