Hi!
Please read the green links that
@Piggies&buns has given you; it is VERY important if you want to have chance at keeping your piggy alive until you can see a vet!
They contain all the practical emergency care support information that you can give your piggy until you can see a vet at the soonest, including how to support a dying piggy if the wait is too long.
Unfortunately, we can't diagnose without a hands-on diagnosis but it sounds like an abscess, cyst or tumour has blown up. From your description it is unclear whether it is connected with a blockage in the nose or not (if it is not connected, perhaps a piece of hay become stuck in one nostril or something else has got into the nose). Heavy sneezing is usually the normal reaction to something getting into the nose, but it sounds rather congested now. Unlike humans, guinea pigs can't breathe through their mouths.
You can try and see whether a bowl of steaming water near her can help ease the breathing a little.
If your piggy is struggling to breathe, it won't have the energy to drink or eat as the need to breathe comes before the need to drink and only thirdly the need to eat. You will have to support feed her, little but often (about every 2 hours during the day and every 3 hours during the night) with mushed up pellets, ideally as close as you can come to 40-60 ml in 24 hours. Keep in mind that around 80% of what a guinea pig should eat in a day is unlimited hay and not veg and pellets, a little piece of fruit (not the healthiest of foods) won't go far.
I am very sorry that this emergency is happening at the most inconvenient of times.
Fingers very firmly crossed!