Hi and welcome!
Please see a vet as soon as you can! One of your piggies has got a fully blown URI (upper respiratory tract infection) and possibly pneumonia and urgently needs an antibiotic. URIs can kill.
Please also ask your vet for a diuretic if the lungs are congested, as well as some mucus dissolving powder to ease the breathing as much as possible, which is vital. Until then, you can place a bowl of steaming water next to the cage. Put 2-3 drops of olbas oils in it if you have that handy; otherwise the plain water will do.
Respiratory infections are sadly not at all rare in young shop piggies in close proximity whose immune system is lowered from all the upheavals to their young lives. You can try and claim back any vet cost from the shop.
Weigh your piggies daily at the same time to monitor the food intake until they are out of the woods. If necessary, you may have to hand feed and hydrate a piggy that has stopped eating or is only eating very little. The need to breathe comes before the need to drink and only thirdly the need to eat.
http://www.guinealynx.info/uri.html
https://www.theguineapigforum.co.uk/threads/complete-hand-feeding-guide.115359/
Have your other piggy checked as well; it is impossible for me to guess what is going on just from your description. It can simply be popcorning form joy or it can be fitting.
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