Hi
More information on ovarian cysts:
Sows: Behaviour and female health problems (including ovarian cysts)
Give your girl time to get over her vet trip and injection but step in with feeding support if she is still not eating by the time you are going to bed.
Keep in mind that three quarters of what a piggy eats in a day is hay which you cannot control by eye but that just 30g of weight loss amounts to merely the difference between a full and an empty belly and the normal weight swing over the course of 24 hours, so I would not panic. We only talk about weight loss from 50g onward.
If needed, you can find all necessary support information via this guide here:
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures
If your girl has indeed a bad reaction, then here is our practical emergency care information bundle.
Emergency Resources and Critical Illness Care - Contents list and subforum link
All the best. She will hopefully get her bearings again. Place some hay close by and leave her be for now.
Here are our tips for vet visits and transport:
Tips For Vet Visits
Travelling with guinea pigs