My guinea pig has been sneezing for a few days and her breathing sounds crackly

Get her to an exotics or small-mammal vet within the day. Guinea pigs with respiratory infections go downhill very fast, and this is the species where "let's give it a week" ends badly. And the hard line first: if she is breathing with her mouth open, if her nose or gums look bluish, or if she has stopped eating altogether, that is an emergency — go now, tonight.

The reason for the urgency is a chain. A guinea pig who feels ill eats less. A guinea pig who eats less shuts her gut down within a day or two. Then you are treating two problems instead of one, and the second one is the one that kills her.

A critical safety point: never give her leftover antibiotics, or anything prescribed for another animal. Several very common antibiotics wipe out a guinea pig's gut bacteria and are fatal to this species. Only a vet who knows guinea pigs should choose the treatment.

Things to check and to raise at the appointment: the bedding — dusty, scented, or pine and cedar shavings irritate airways, and ammonia from a cage that is overdue for a clean does real damage, so put your nose down at her level for a minute and be honest about what you smell. The new bedding brand is worth mentioning. Also the hay quality (dusty hay is a common culprit), the humidity, whether her cage-mate is sneezing too, and any new animal in the last few weeks.

The most useful thing you can do today, and it is genuinely powerful: weigh her every day on a kitchen scale, in grams, and write it down. A guinea pig's weight starts falling before anything else is visible; that daily number is the best early-warning system a guinea pig owner has. Keep her warm, keep hay in front of her, and don't delay the appointment to watch and see.